New York City Mayor Bill de Blasio and other city officials said investigators had ruled out a natural gas leak as the origin of the blast, but they stopped short of calling it a bombing and declined to specify precisely what they believed may have triggered the explosion.
Police said a sweep of the neighborhood following the blast turned up a possible "secondary device" a short distance away.
CNN, citing law enforcement sources, reported that it appeared to be a pressure cooker with wires attached to it and connected to what resembled a cell phone. A piece of paper with writing on it was found nearby, according to CNN's account.
Remaining circumspect about the exact nature of the actual explosion, De Blasio said early indications were that it was "an intentional act." He added that the site of the blast, outside on a major thoroughfare in the fashionable New York City district, was being treated as a crime scene.
"There is no evidence at this point of a terror connection," the mayor said at a news conference about three hours after the blast. He added, "There is no specific and credible threat against New York City at this point in time from any terror organization."
The mayor also said investigators did not believe there was any link to a pipe bomb that exploded earlier on Saturday in the New Jersey beach town of Seaside Park. No injuries were reported in that blast, in a plastic trash can along the route of a charity foot race. Authorities said they believed it to be a deliberate act.
But a U.S. official said that a Joint Terrorism Task Force, an interagency group of federal, state and local officials, was called to investigate the Chelsea blast, suggesting authorities have not ruled out the possibility of a terror connection.
A joint task force also took the lead in investigating the New Jersey incident.
ONE PERSON SERIOUSLY INJURED
A law enforcement source said an initial investigation suggested the Chelsea explosion occurred in a dumpster but the cause was still undetermined. CNN cited law enforcement sources as saying they believed an improvised explosive device caused the blast.
President Barack Obama, who was attending a congressional dinner in Washington, "has been apprised of the explosion in New York City, the cause of which remains under investigation," a White House official said. "The president will be updated as additional information becomes available," the official added.
New York City Fire Commissioner Daniel Nigro said 29 people were hurt in the blast, and 24 of them had been taken to area hospitals, including one person he described as seriously injured. The rest suffered various cuts, scrapes and other minor injuries from shattered glass and other debris, Nigro said.
The explosion, described by one neighbor as "deafening," happened outside the Associated Blind Housing facility at 135 W. 23rd Street. The facility provides housing, training and other services for the blind.