DEVELOPING: An explosion that injured at least 29 people in New York City late Saturday appeared to be "intentional," Mayor Bill de Blasio announced, as sources told Fox News an apparent explosive device -- a pressure cooker -- was found four blocks away.

The blast, on West 23rd Street in the city's Chelsea neighborhood, appeared to originate from a construction toolbox in front of a building, a law enforcement official told The Associated Press. A garbage can was found mangled nearby. Police would not give any more details about the device found on West 27th Street.