The fatal shooting of five people, including a pregnant woman, at a Pittsburgh garden party was a planned, calculated attack, prosecutors have said.
One of the gunmen used a handgun to steer the victims towards the house's porch, said Allegheny County district attorney Stephen Zappala, where a second gunman picked them off with an AK-47-style rifle. He said the victims were shot in the head.
The five were killed late on Wednesday during the party in Wilkinsburg. Two others are in a critical condition.
Mr Zappala said as many as 15 people had been at the party and that police are searching for at least two suspects. The motive is not known.
Earlier, a medical examiner released the identities of the five people killed in the shooting.
The victims were 35-year-old Jerry Shelton, 37-year-old Tina Shelton, 27-year-old Brittany Powell, 25-year-old Chanetta Powell and 26-year-old Shada Mahone.
The medical examiner said Mr Shelton and the Powells were siblings.
A neighbour said he saw at least three children run onto the porch screaming “Mommy! Mommy!” after the shooting.
Robert Morris, a 15-year-old who lives across the street from the home, said it was "terrible".
He and his father, Carl Morris, said a black Labrador retriever that had been shot in the tail ran to their house after the shooting.
They said a young woman and her child lived at the home.
Another neighbour, Mike Jones, said violence is rare in the neighbourhood.
The Allegheny County Police Department said the motive for the fatal shootings was still unknown.
A 41-year-old woman who was wounded in the shooting was treated and released from a local hospital, according to a hospital spokesman. Two males remain hospitalised in critical condition, officials said.
One of the dead, Tina Shelton, was a mother of five who held down three jobs and was encouraged by family members to attend the barbecue as a way to relax after work, her father Vernes Pugh and other relatives told local station WPXI-TV.
The killings in Wilkinsburg, a borough of about 15,000 mostly lower and middle-income residents, is the latest in a series of mass shootings that have made gun control a matter of heated debate in the US.
"Wilkinsburg is a community filled with grief, shock and anger this morning," Allegheny County Executive Rich Fitzgerald said in a statement on Thursday.
“As a community, we must say enough is enough, and providing information on this senseless shooting can be our first step to stopping the violence in our communities.”
The shooting generated more than 10,000 messages on Twitter, with many expressing outrage over gun violence.
"Woke up to the terrible news about the Wilkinsburg shooting," US congressman Mike Doyle, who represents the Pennsylvania district that includes Wilkinsburg, tweeted.
“My prayers are with the families and friends of the victims.”
Reuters