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, Reuters reports.
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."