Nearly all of the evidence against the two 14-year-old boys convicted of the murder of Ana Kriégel this week was circumstantial, a term often used to imply that it is weak. But the trial of Boy A and Boy B showed the opposite is often the case. Pieces of circumstantial evidence can together paint a compelling picture that leaves a jury in no doubt about the guilt of an accused.

The teenagers, who on Tuesday became the youngest people in the history of the State to be convicted of such a crime, killed Ana at Glenwood House in Lucan, on the western edge of Co Dublin, on May 14th, 2018. She was 14; they were then 13. Boy A had pleaded not guilty to murder and sexual assault involving serious violence. Boy B had pleaded not guilty to murder. Their names were not revealed because of their age.

Circumstantial evidence is any evidence that implies the truth of a fact rather than supporting it directly. The testimony of a witness to a murder is direct evidence. The testimony of a witness who saw the suspected killer leave the crime scene is indirect or circumstantial evidence.

So the presence of Ana Kriégel’s blood on the boots of Boy A implied he was at the scene of her murder. CCTV footage of Boy B walking with Ana towards the abandoned house where she was killed implied he went there with her. The lies and evasiveness of the boys implied they had something to hide.

A jury would never be asked to convict on a single piece of circumstantial evidence. The Garda and prosecution needed to gather many pieces to allow them to meet the burden of proof.

Dozens of strands of circumstantial evidence pointed towards the guilt of Boys A and B, but five key pieces stood out and made a conviction possible.

1. The boots

Gardaí had suspicions about Boys A and B even before Ana’s body was found, on May 17th, 2018. Two days earlier, during the search for Ana, investigators got the boys to show them the route they claimed to have walked with Ana in St Catherine’s Park, near her home in Leixlip, in Co Kildare, the day she went missing. The gardaí noticed that the boys appeared confused at one point and that they exchanged a look or glance.

Once Ana’s body was found the investigation immediately focused on the teenagers. They had been the last people to have been seen with Ana on May 14th, but the Garda had nothing to place them at the scene of the crime.

Detectives needed a solid link to one or both. They already had Boy A’s clothes and boots. When he first spoke to gardaí, Boy A claimed he had been beaten up by two men in the park shortly after leaving Ana’s company. He said this was how his face, arm and knee were injured.

The Garda’s priority was to find Ana, but it was also obliged to investigate the allegations of assault. Det Garda Gabriel Newton therefore called to Boy A’s house for his clothes and boots, in the hope that they would yield clues about his attackers.

It quickly became apparent that the two men were fictional. Nobody in the park that day had seen anyone matching their description. Nor had they been spotted on the 700 hours or so of CCTV footage gardaí had watched.

But the existing investigation meant that, when Ana’s body was found, detectives did not need a warrant to get the clothes Boy A was wearing that day.

Forensic testing showed Ana’s blood in nine locations on his boots. In the words of John Hoade, a forensics expert, this proved that Boy A “either assaulted Anastasia Kriégel or was in very close proximity to Anastasia Kriégel when she was assaulted”.

Gardaí now had more than enough evidence to arrest both Boy A and his best friend, Boy B, and bring them in for questioning. They were also granted search warrants for both teenagers’ homes.

Many of Boy A’s responses in interview were that he had no comment. But when gardaí showed him a photograph of the boots, and told him Ana’s blood had been found on them, he asked if the detective was joking. “Are you actually being serious?” he said. Det Garda Tomas Doyle said he wouldn’t joke about something like that.

Boy A asked if he could leave and get some air. His solicitor asked if he was feeling sick, and someone got the boy a glass of water. Det Garda Doyle told Boy A this was a very serious matter. “I am aware,” the boy responded.

The boots were the first building block in what the prosecution called the overwhelming forensic case against Boy A.

During the trial, in the absence of the jury, his defence would attempt to prevent the boots being introduced as evidence. Patrick Gageby, Boy A's barrister, argued that they had been obtained under false pretences and that Det Garda Newton was uninterested in investigating the alleged assault and interested in the case only as it related to Ana.

He asked the detective why she would seize clothes when they had already been washed. Clothing can still yield forensic clues after washing, Det Garda Newton responded.

The gambit failed. Mr Justice Paul McDermott said gardaí took the boots and clothes as part of the investigation into a serious assault. It was simply luck on their part that the boots went on to yield vital evidence in the murder investigation.

2. The ‘murder kit’

The backpack found in Boy A’s bedroom after his arrest, on May 24th, became known almost immediately as the “murder kit”. It is easy to see why. The distinctive bag contained knee pads, shinguards, gloves, a “snood” scarf and a skull-shaped mask.

The mask stood out to detectives. Skin-coloured, it covered the face only down to the upper jaw. Holes had been cut for the eyes and nostrils, and fangs had been cut into the bottom of it and painted red, to make them look bloodstained. In court it was darker than it had been when gardaí found it, because of a dye used to extract fingerprints from it.

Despite its chilling appearance, the mask had innocent origins. It was made as part of a school project, and Boy A had used it as a Halloween mask. Mask-making notes in a copybook in his room seemed to support this, as did searches on his mobile phone for examples of masks.

Asked about Boy A’s mask by gardaí, Boy B referred to it as a zombie mask. He said that it was “really cool” and that he had worn it before himself.

Asked why he called it a zombie mask, Boy B said that was what Boy A called it.

Detectives were also interested in the gloves in the backpack. Although they appeared innocuous, the gloves provided an important clue, explaining why no fingerprints were found in Glenwood House.

They also appeared to match the gloves Boy A was seen wearing when he was captured on CCTV walking through St Catherine’s Park towards the abandoned house the day Ana was murdered. Detectives had wondered why he was wearing woollen gloves on a warm summer evening.

The backpack matched the one Boy A was spotted wearing that day. He wore it whenever he went outside, Boy B said.

On their own these items were unusual but proved nothing. Yet they were suspicious enough for gardaí to send them for forensic testing, leading to a breakthrough. Ana’s blood was found on both the inside and the outside of the mask, and Boy A’s DNA was found around the nose and mouth. Ana’s DNA was also found on the knee pads and gloves, and on the inside and outside of the backpack.

Gardaí had already forensically linked Boy A to the crime scene through his boots. But the presence of Ana’s DNA on the other items showed it was more than an assault; it was an extremely violent attack that resulted in a huge loss of blood. It showed the crime was murder, not manslaughter.

Perhaps more importantly, the mask, shinguards and knee pads showed an element of planning. Boy A packed those items that day because he knew Ana was likely to fight back. Boy A was tall for his age, but Ana was taller.

All these items would be shown to the jury during the trial. But the prosecution wanted to go a step further. During the investigation Hoade, the forensics expert, had dressed a mannequin in the items, as well as in the clothing worn by Boy A on the day of the murder.

A dummy dressed in a skull mask and a face scarf with a hood over its head was a chilling representation of one of the last things Ana would have seen before losing consciousness.

Brendan Grehan SC, the prosecuting counsel, wanted to show the jury photographs of the mannequin. He said it would be nothing more than a visual aid, to show them how items from the backpack were intended to be worn. He said the dummy was “no more than a representation of what the jury has already seen, in a different format”.

Gageby, for Boy A, objected on the basis that the mannequin was speculative and there was no evidence it accurately portrayed what was worn at the time. There was no evidence to show Boy A wore his hood up during the attack, for example.

Mr Justice McDermott ruled against the prosecution. “Whatever limited probative value is outweighed by the disproportionate prejudicial effects,” he said. “I’m not satisfied that this photo should go in.”

The photographs would have been a striking exhibit, but they were not required. The forensics experts' cold, dispassionate evidence about their findings conveyed the information just as effectively.

3. The tape

Tescon tape, used for attaching insulation to roofs, is highly specialised. A roll costs about €30. “It’s not something you would buy on a whim,” Adrian Crosson of Ecological Building Systems said. “You buy it for a specific purpose.”

Crosson’s company is the only supplier of the tape in Ireland, and orders it only on request.

A long length of Tescon tape, which is highly adhesive, was found wrapped around Ana’s neck when her body was found. She had her fingers inside it, as if she had been trying to pull it off, and a piece of her jewellery was stuck to the adhesive side.

Despite what confronted the Garda search team at 1pm on May 14th, 2018, when they entered Glenwood House, it was later established the tape had not been used to strangle the girl.

The State pathologist at the time, Prof Marie Cassidy, found Ana died from blunt-force trauma to the head. Strangulation may also have contributed to her death, but this had been most likely done by hand. If the strangulation had been caused by the tape, there would have been marks all around the neck instead of just on one side.

Gardaí believed the tape had instead been used to drag Ana to the other side of the room, over to where there was more light. It could not be determined if she was conscious or alive at this point.

Although the tape helped little in determining how Ana died, it was vital in establishing an important link between the two boys. And its presence suggested planning. An accused could explain bringing a scarf or gloves for a walk in the park. Bringing specialised insulation tape was another matter.

When gardaí searched Boy B’s home they found the same brand of tape in a shed. When detectives asked him about this, Boy B said he used it to make grips for weapons. He explained that he liked to make things and had made his own bow and arrow, which he used for target shooting in his back garden.

“I also like making other stuff, but usually when I make something I use that tape.”

Detectives showed the boy a photograph of the same brand of tape found at the murder scene.

“Wait a minute. Holy s**t. Oh my God,” Boy B said. “I gave [Boy A] tape a couple of weeks ago.”

He said Boy A wanted the tape because he was making a “big pole kind of weapon. So I gave him a used-up roll, half-used-up roll. I gave it to him. Here it is. Construction tape.”

Boy B denied bringing the tape to the abandoned house himself.

Boy B’s story was backed up by his father, who told the trial he was in the garden with Boy B the weekend before Ana’s murder when his son told him he had lent Boy A the tape.

The father was furious. The tape was expensive and so sticky that it was very difficult to handle. “I became very frustrated. I told him he can’t give anything from my shed to anyone without my permission.”

We don’t know how truthful Boy B was being when he described how he lent Boy A the tape.

Aside from his father’s testimony there was other evidence to support Boy B’s account. An analysis of the torn edge of the tape found at the scene showed it did not match the marks on the tape roll found in the shed.

This indicated that a different roll was used or, more likely, that another bit of tape had been removed from the roll before the part found at the scene. This lent support to Boy B’s claim that Boy A had used the tape for another purpose before the murder.

But gardaí still suspected he had brought the tape to the house that day. Boy B was seen carrying a backpack on May 14th. He told gardaí he used it to carry his water bottle. If so, why did Boy B claim he stopped behind the ranger station in St Catherine’s Park to get a drink of water, they wondered.

For the prosecution, anything that showed a level of premeditation by Boy B was significant, especially as there was no forensic case against him.

The prosecution could not state definitively that Boy B had brought the tape to the house or that he knew what Boy A planned to do with it. But it could present the facts and let the jury make up its own mind.

4. The CCTV

St Catherine’s Park is a well-equipped amenity that locals value highly. They have few concerns about its safety, and children play freely throughout its 80 hectares, or 200 acres. This is partly because the park is well covered by CCTV, including movement-activated cameras that can follow subjects as they walk past.

After Ana was reported missing, gardaí combed through footage from every camera in an effort to trace her movements. They also obtained footage from many other sources, including private cameras on the sides of houses, and even checked footage from buses and trains that passed nearby that day.

In terms of finding Ana alive, it was a fruitless exercise. Detectives later determined she was murdered within 40 minutes of leaving her home, just a few kilometres from her front door.

But the footage was invaluable in the investigation of her death. Gardaí were able to observe Boys A and B walking home from school together and, an hour later, Boy B walking to Ana’s house.

They observed Boy B leading her away from the house – “effectively the last definitive sighting of Ana alive”, the prosecution later called it.

Minutes later, cameras in the park picked up two figures assumed to be Ana and Boy B walking near the BMX track in the general direction of Glenwood House.

Crucially, the cameras also picked up Boy A walking in the same direction about 20 minutes earlier. This contradicted the account of Boy B, who claimed he collected Ana and “handed her over” to Boy A a little way into the park before leaving them to talk.

The footage supported the theory that Boy A had gone ahead to Glenwood House and was lying in wait for Ana there.

No CCTV footage showed Boy A going into Glenwood House, but a witness did see a teenager matching his description climbing into the field beside the house.“What struck me was he didn’t go through the field at a 90-degree angle. His track was a 45-degree angle to the road, and it was a beeline towards the disused farmhouse. I assumed this was a schoolboy taking a short cut home,” Gerard Redmond told the jury.

The mass of CCTV evidence presented the prosecution with a challenge. It needed to find a way of presenting it to the jurors coherently and without overwhelming them.

It was decided to present the footage chronologically, and Grehan and Garda Seamus Timmins provided commentary as it played on the courtroom’s large flat-screen televisions. They also used a digital map to show where each piece of CCTV footage came from. It was an innovative method of showing the jurors the exact route each person had taken without having to bring them to the park.

5. The Kriégels’ testimony

Some of the first evidence the jury heard was from Ana’s parents, Geraldine and Patric Kriégel, who were calm and composed and never showed any anger – although Geraldine appeared frustrated when Gageby suggested Ana might have had trouble controlling her impulses.

She responded that Ana “wouldn’t hurt a fly” but might sometimes throw pillows around the room in frustration. “I wasn’t hugely concerned about that,” she said.

The parents’ evidence served several important purposes. First, and most obviously, it laid out to the jury the timeline of Ana leaving the house that day and their efforts to find her.

Ana left home at about 5pm with Boy B, Patric said. She was smiling, and he believed her when she said she wouldn’t be long.

Geraldine described becoming “immediately concerned” when she got home, 20 minutes later, and found out Ana had left with Boy B. For the next four hours they searched for Ana and tried to find out where Boy B lived. At 9pm they went to Leixlip Garda station, which was able to locate his address.

Much of their evidence dealt with the difficulties Ana faced in the final months of her life: her loneliness, her need for friendship and the relentless bullying she endured.

Much of the bullying was about her height, her mother said. Ana was strong and tall, “a typical Siberian”, Geraldine said. She was also bullied about being adopted: other children told her she had “fake parents”.

There was palpable public anger outside the courtroom that this sensitive evidence was being elicited. Why did Boy A and Boy B enjoy anonymity while Ana’s most personal matters were aired in court?

But there was a good reason, the prosecution said. They needed to convey Ana’s vulnerability and need for friendship, because this is what her killers exploited to lure her to Glenwood House. Ana’s face lit up when Boy B called to the door to tell her his friend wanted to see her. She “bounded” out of the house, the prosecution said.

The final reason Geraldine and Patric Kriégel’s evidence was important was that it humanised Ana. All too often in murder trials the victims become an abstract piece of evidence.

Ana’s parents’ description of her personality, hobbies and interests made her a real presence for the jurors, and perhaps allowed them an insight into the thoughts going through her head on her last day alive.