Investigators sifted through blast remnants, examined video and scoured the scene of an explosion that wounded 29 people in Manhattan, attempting to establish if there were any links to international terrorism.
The explosion on a commercial and residential street in New York City's Chelsea district last night sent a deafening roar and a powerful shock wave through several blocks, wounding people with shrapnel and flying glass.
All 29 victims were released from hospital, officials said today.
Mayor Bill de Blasio pleaded for any witnesses to provide tips and promised a security presence that would be "bigger than ever" for the United Nations General Assembly bringing together world leaders in Manhattan for six days starting on Tuesday.
FBI investigators will examine remnants of the bomb plus an unexploded device found four blocks away as well as a pipe bomb that exploded about 130km away in New Jersey yesterday to see if they were connected, officials said.
Police recovered video from both scenes in Manhattan including images of the explosion itself, New York Police Commissioner James O'Neill said.
New York police, the FBI and the US Bureau of Alcohol, Tobacco, Firearms and Explosives converged on the site for their first daylight view of the site of the explosion, cordoning it off and placing dozens of evidence markers on the ground. Police closed several surrounding blocks to traffic.
"We are in the middle of a very complex post-blast investigation," Commissioner O'Neill said.
Although no international group had claimed responsibility, New York state Governor Andrew Cuomo said detonating a bomb in New York City "is obviously an act of terrorism".
But the mayor resisted when reporters pressed him to call the blast an act of terrorism, saying investigators had yet to determine if there was a political motivation.
"It was intentional. It was a violent act. It was certainly a criminal act. It was a bombing. That's what we know," said Mr de Blasio, flanked by high-ranking officials of the FBI and the city police and fire departments.
"It could have been something personally motivated. We don't know yet," Mr de Blasio said.
The mayor and the governor both promised New Yorkers would not be cowed and that apart from the street closures life would continue as normal.
New Yorkers, be vigilant. We are the most protected city in the world. You will see a very substantial NYPD presence today & this week.
— Bill de Blasio (@BilldeBlasio) September 18, 2016
A sweep of the neighbourhood following the blast turned up another device four blocks away consisting of a pressure cooker with wires attached to it and connected to a mobile phone.
Pressure cooker bombs were used in the 2013 Boston Marathon bombing that killed three people and wounded more than 260.
The New Jersey explosion came from a pipe bomb, officials said. A US official said the motive remained unknown and insufficient evidence had been gathered to link the two New York bombs.
There was no evidence to connect them to the New Jersey blast, said the source.
"Almost anybody could have fabricated these bombs and used cell phones as timed detonators," said another US official familiar with the inquiry.
"There are instructions all over the internet, and the crudity, positioning, and relative ineffectiveness of these does not suggest that a more sophisticated group played any role in this."
"When you see the damage," Mr Cuomo said, "I think we were fortunate that there were no fatalities."
So-called Islamic State claims shopping centre attack, says agency
Meanwhile, a so-called Islamic State supporter reportedly carried out the stabbing attack that wounded at least eight people at a mall in the US state of Minnesota yesterday, the militant group's Amaq news agency said.
"The executor of the stabbing attacks in Minnesota yesterday was a soldier of the Islamic State and carried out the operation in response to calls to target the citizens of countries belonging to the crusader coalition," Amaq said in a statement.
Reuters was not immediately able to verify the authenticity of the claim.
A man wearing a private security uniform and armed with at least one knife stabbed eight people at the Crossroads Centre mall before he was shot dead by an off-duty police officer, authorities said.
