New York Governor Andrew Cuomo says the explosion in Manhattan that injured 29 people was likely an act of domestic terrorism, and not linked to Islamic State.
"A bomb exploding in New York is obviously an act of terrorism but it's not linked to international terrorism, in other words, we find no Isis connection," Mr Cuomo said in a press conference on Sunday (local time).
FBI agents are examining remnants of the bomb plus an unexploded device found four blocks away and another device that exploded in New Jersey on Saturday to see if they are connected.
"We know it was a very serious incident, but we have a lot more work to do to be able to say what kind of motivation was behind this," New York Mayor Bill de Blasio told local media. "Was it a political motivation? Was it a personal motivation? We do not know that yet."
About 1000 additional state police and national guard officers were being sent to the city to patrol transportation hubs as a precaution, Mr Cuomo said.
The FBI was taking evidence from all three sites to its special crime laboratory in Quantico, Virginia, he said, adding that he had been briefed by "all the relevant agencies".
Newshub's Patrick Gower, speaking to Paul Henry from outside Manhattan's UN building, said there does not appear to be extra security after bomb attack, but when US President Barack Obama and other world leaders arrive, the area will be in lockdown.
"We will find who planted these explosives and they will be punished. We will not allow these type of people and these type of threats to disrupt our life in New York," Mr Cuomo said.
Most of the injured were treated for minor injuries including shrapnel wounds, though police listed one injury as serious.
Mr Cuomo said all had been released from hospital.