A gas line explosion at a New York City high school Thursday night injured three construction workers, one critically. At least three floors of the building were badly damaged, authorities reportedly said.

The incident took place at John F. Kennedy High School in New York's Bronx borough when seven people were carrying out construction work in a laboratory on the sixth floor. Nearly 140 firefighters reportedly responded to the scene.

"This was a very troubling evening for residents here in Marble Hill," New York City Mayor Bill de Blasio reportedly said at a news conference Thursday night.

“The sixth floor had tremendous damage. There will be a thorough investigation,” he reportedly said, adding that the injured workers had "very serious burns" and were being treated at local hospitals.

An official who reportedly spoke to the injured workers said “a contractor was doing work with a blow torch and must have hit a gas line.” However, it was unclear what triggered the explosion.

The mayor reportedly said that the building housed eight schools with over 4,000 students, but he couldn't confirm if the damage would impact the start of school on Sept. 9.

"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, according to the Associated Press.