Below is a timeline of the main events in New York overnight. All times are local.
1.15am
Donald Trump may have moved ahead of New York City officials when he declared Saturday evening that a "bomb went off" in the city before officials had released details.
Video shows moment of IED Explosion at W 23 St & 6th Ave in Manhattan pic.twitter.com/O270bZWOzF
— New York City Alerts (@NYCityAlerts) September 18, 2016
Trump made the announcement minutes after stepping off his plane during a rally at an airport hangar in Colorado Springs, Colorado. He told the crowd, "we better get very tough, folks".
The Republican presidential nominee made the comments around 9.10pm, shortly after the explosion in Manhattan's crowded Chelsea neighbourhood and as emergency officials were responding to the blast.
Trump's rival Hillary Clinton, meanwhile, sought to present a more cautions response, underscoring the difference between the two candidates' styles.
Clinton was briefed on the incidents shortly after her speech to the Congressional Black Caucus Foundation dinner in Washington.
12.30am
A law enforcement official says a second device officers are investigating a few blocks from the scene of a Manhattan explosion appears to be a pressure cooker attached to wiring and a mobile phone.
The source, speaking on condition of anonymity because the person was not authorized to speak about an ongoing investigation, says the device was found inside a plastic bag on West 27th Street.
Police are advising residents of the block where the device was found to stay away from windows facing 27th Street.
The pressure-cooker device was found four blocks from the location on West 23rd Street where an explosion injured more than two dozen people Saturday night.
11.55pm
Hillary Clinton says she has been briefed about explosions in New York and New Jersey and a mall attack in Minnesota and is calling on the nation to support first responders and to pray for victims.
Police in New York City are investigating an explosion that injured more than two dozen people.
In New Jersey, a pipe bomb exploded in Seaside Park shortly before a 5k run but caused no injuries.
Police in St. Cloud, Minnesota, are investigating reports of multiple injuries at a mall possibly involving both shooting and stabbing.
Clinton tells reporters travelling with her that she will have more to say when more is known about the incidents.
Both Clinton and her opponent, Donald Trump, are referring to the explosion in New York City as a bombing, a description authorities did not use publicly in the early hours of the investigation.
Trump had just arrived in Colorado Springs, Colorado, for a rally on Saturday night when he said "a bomb" had gone off in New York City and that "nobody knows what's going on."
11.45pm
New York City mayor Bill de Blasio says there is no reason to think that an explosion in Manhattan’s Chelsea neighbourhood had a terrorist connection, but he did call the blast an “intentional act.”
The mayor also said there is no specific connection to a pipe bomb explosion in the New Jersey shore town of Seaside Park earlier Saturday at a charity run. There were no injuries in that incident.
Fire Commissioner Daniel Nigro says a total of 29 people were injured in the Chelsea blast. He says one person's injuries were serious and the rest were minor.
Police Commissioner James O’Neill says officers are investigating a possible second explosive device a few blocks away from the explosion at about 8.30pm on West 23rd Street.
11pm
A law enforcement official says an explosion in Manhattan’s Chelsea neighbourhood appears to have come from a construction toolbox in front of a building.
The official spoke on condition of anonymity because the person was not authorized to speak about an ongoing investigation.
Police say 26 people have sustained minor injuries in the explosion on West 23rd Street.
Mayor Bill de Blasio and police officials are expected to brief reporters at the scene.
The blast happened in front of a residence for the blind. Witnesses say the explosion at about 8.30pm blew out the windows of businesses in the area.
10.20pm
Police and fire-fighters are at the scene of a possible explosion in New York City, and authorities say 25 people suffered minor injuries.
Police spokesman J. Peter Donald said on Twitter the possible explosion happened at about 8.30pm Saturday on West 23rd Street in the Chelsea section of Manhattan. He says several people were taken to hospitals with injuries. The Fire Department tweeted that none of the injuries appear to be life-threatening.
No detail about the extent of damage was immediately available.
Chris Gonzalez, visiting from Dallas, was having dinner with friends at a restaurant. The 26-year-old says she heard and felt a blast. She said the restaurant went quiet.
Witnesses say FBI and Homeland Security officials are also at the scene.
The reports of a possible blast come hours after a pipe bomb exploded in Seaside Park, New Jersey, shortly before thousands of runners participated in a charity 5K race to benefit Marines and sailors.
9.40pm
Police and fire-fighters are at the scene of a possible explosion in New York City.
The Fire Department says the blast was reported shortly before 9pm Saturday on West 23rd Street in the Chelsea section of Manhattan.
No information about injuries or about the extent of the damage was immediately available.
City Councilman Corey Johnson, who represents the area, told cable news station New York 1 that FBI and federal Homeland Security officials were on the scene, in addition to New York City police.
Numerous emergency vehicles are in the area, which is a major thoroughfare with many restaurants.
The reports of a possible blast comes hours after a pipe bomb exploded in Seaside Park, New Jersey, shortly before thousands of runners participated in a charity 5K race to benefit Marines and sailors.
9pm
Firefighters are at the scene of a possible building explosion in New York City.
The Fire Department says the blast was reported shortly before 9pm Saturday on West 23rd Street in the Chelsea section of Manhattan.
No information about injuries or about the extent of the damage was immediately available.
Agencies / AP