Twenty-nine people were injured in an explosion in the Chelsea area of Manhattan on Saturday night.
Another suspicious device was later found nearby and removed by authorities.
New York Mayor Bill de Blasio called the blast, which happened at about 8.30pm, an "intentional act".
He said: "Tonight, New York City experienced a very bad incident. We have no credible and specific threat at this moment."
De Blasio said the explosion had no terrorist connection and wasn't related to a pipe bomb explosion earlier on Saturday at a charity run in New Jersey.
It's not clear at the moment who was behind the blast and what motivated it.
A law enforcement official told The Associated Press that a second device officers investigated appeared to be a pressure cooker attached to wiring and a mobile phone.
The source said the device was found inside a plastic bag on West 27th Street.
The device was removed with a robot and taken to the department firing range in the Bronx, officials said.
When officers were investigating the second site, police spokesman J Peter Donald tweeted a warning to residents to stay away from windows facing 27th Street.
The law enforcement official also said the explosion appears to have come from a construction toolbox in front of a residence for the blind, on West 23rd street. Photos from the scene show a twisted and crumpled black metal box.
Witnesses say the explosion blew out the windows of businesses and scattered debris in the area.
Officials said no evacuations were necessary but Fire Department Commissioner Daniel Nigro said several people were taken to hospitals with injuries.
One person suffered a puncture wound that was considered serious but he said the other injuries were minor, described as scrapes and bruises.