NEW YORK (CNN) -- At least 21 people were hurt by what police and fire officials described as an explosion Thursday at an eight-story technical school in Manhattan.
The incident happened at Apex Technical School in the Chelsea neighborhood.
Streets in the area around 19th Street and Sixth Avenue were closed to traffic.
At least 21 people, some with cuts visible, were being treated at the scene.
Police, fire and rescue units responded to the blast, and some rubble was visible on the street. But no fire or smoke was evident.
More than 100 firefighters were on the scene, fire officials said.