NEW YORK » A gas explosion during construction at a New York City high school Thursday night seriously injured three workers and caused heavy damage to several floors of the building.

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

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

New York City Mayor Bill de Blasio said work was being done by private contractors to improve the school’s science labs.

He said the Department of Buildings was investigating the structural integrity of the building, which suffered major damage. It wasn’t immediately known what triggered the gas explosion.

A 73-year-old man who lives across the street from the school told The New York Times he ran outside when he felt the force of the blast.

“The whole building shook for a very short time, like an earthquake,” said Eric Kumaga, adding that he saw no fire or smoke.

The mayor said the building housed eight schools with over 4,000 students, but he couldn’t confirm if the damage would impact the start of school.

“If part of the building can be opened, if all of the building can be opened, we won’t know until that full assessment is done,” de Blasio said.

The first day of school is Sept. 9.