New York City Mayor Bill de Blasio and other city officials said investigators had ruled out a gas leak as the cause 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 neighbourhood following the blast had turned up a possible "secondary device" four blocks away consisting of a pressure cooker with wires attached to it and connected to a cell phone.
The latest blast came less than a week after law enforcement agencies around the country were on heightened alert for the 15th anniversary of the Sept. 11, 2001, airline-hijacking attacks that killed nearly 3,000 Americans in New York, Washington and Pennsylvania.
Remaining circumspect about the exact nature of the explosion in Chelsea, 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 lower West Side Manhattan neighbourhood, was being treated as a crime scene.
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, from a device planted in a plastic trash can along the route of a charity foot race.
