NEW YORK (CBS) -- 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. CBS New York reported the blast occurred at 8:09 p.m. at John F. Kennedy High School in the Marble Hill section of the Bronx. Nearly 140 firefighters responded to the scene. The FDNY said three workers were hurt in the explosion. One was reported in critical condition; the other two suffered serious but non-life-threatening injuries. All three were taken to the burn unit at Jacobi Medical Center. 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 custodian who was working at the time of the explosion told CBS Radio station 1010 WINS' Gary Baumgarten what happened. Photos at the scene showed windows blown out and other interior and exterior damage. "There's contractors up there. They're working up there. So they're working up there -- I think they're making them do a few classrooms or something like that; chemistry room. And it just broke down when they hit it with a blow torch, and that was it," he said. The custodian said he smelled natural gas shortly before the explosion, but there was no smoke or fire. The mayor said people around the neighborhood felt the blast. "A very troubling evening for residents here in Marble Hill. They felt a blast. They felt a building shake. I talked to some of the elected officials here -- they literally felt it themselves," de Blasio said. "And what we see here at JFK High is a shocking scene." "The room I was in shook. The whole room shook," resident Emily Langer told CBS New York. "You feel that, 'Oh my God.' We went outside. And our neighbors were like, 'Oh my God, did you feel that? What was that?' And then right away, we heard the sirens." Neighbors said they thought the worst when the heard the loud boom. "We thought it was a terrorist attack," said Jackie Morris. "We really did not know what it was." The school serves more than 1,200 students in grades 9-12. The first day of school is Sept. 9. The mayor said he couldn't confirm if the damage would impact the school's start date. "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. Follow us on Twitter @Local12 and LIKE us on Facebook for updates!
