A gas explosion during construction at a New York City high school has seriously injured three workers.

Fire officials say it happened Thursday night at the John F. Kennedy High School in the Marble Hill section of the Bronx. Nearly 140 firefighters responded to the scene.

Authorities say construction crews were working on a sixth floor gas line when the explosion occurred, seriously burning the contractors.

New York City Mayor Bill de Blasio says the workers had been hired to build science labs. He says the Department of Buildings is investigating the structural integrity of the building, which suffered major damage. It wasn't immediately known what triggered the gas explosion.

The school serves more than 1,200 students in grades nine through 12. The first day of school is Sept. 9.