NEW YORK (CBSNewYork/AP) -- Fire marshals have determined that a gas explosion that damaged a school and injured three construction workers was sparked by a lit match.
The Thursday night blast caused heavy damage to several floors of the John F. Kennedy High School, at 99 Terrace View Ave. in the Marble Hill section of the Bronx.
One of the workers lit a match to test a new gas line that was being installed in a science lab at the school, causing a "massive blast," Mayor Bill de Blasio first told 1010 WINS Friday afternoon.
"I don't pretend to be an expert, but I do not believe this is standard procedure. I believe this was a mistake and obviously a very costly one for the three workers involved and for everybody else that's been affected," de Blasio said. "We'll get a fuller sense of things as we put more pieces together but it certainly does not appear to be something that should've been done."
1010 WINS Interview With Mayor De Blasio On JFK H.S. Explosion
The three workers were seriously burned. Two are listed in serious condition and one is in critical condition, CBS2's Janelle Burrell reported.
"Our hearts go out to the three workers who were very, very seriously injured in this situation," de Blasio said, commending the FDNY for its swift and large response. "These three workers clearly would've lost their lives if the FDNY had not gotten there so quickly and done such a great job."
Thirty-one FDNY units, 138 firefighters responded to the scene.
Blast Leaves 'Very Serious Damage' At Bronx High School
As CBS2's Alice Gainer reported, the building, which houses eight schools for over 4,000 students, will not be ready for the first day of school on Sept. 9, the mayor said.
"The JFK building will not be reopened in time for opening day of school, there will be more work to be done," de Blasio said. "We're now going to work to make sure that every student has a good and appropriate place for school."
Students at two charter high schools that were supposed to start class on Monday will be relocated to the In-Tech Academy in the Bronx.
"We will never allow students to go back into a building that is not totally safe, so right now we're concentrating on fixing those three floors, and investigating the cause of this issue," said schools Chancellor Carmen Fariña.
There is no timetable for how long repairs will take to the JFK building.
Meanwhile, it is unclear if criminal charges could be filed.
"Clearly there was no negative intent here, but clearly this was an avoidable accident from what we're seeing so far and something we're going to learn from," de Blasio said. "We're going to take a very close look at what happened here and see if there's some important lessons to learn but I don't want to speculate on charges."
Blast Leaves 'Very Serious Damage' At Bronx High School; May Impact Start Of Classes
Staff at the school said students were inside the building just hours before the blast.
"It was really scary. It's nothing to take lightly," said school employee Shanice Seignious. "If this happened earlier, there was about 200 kids there at the school yesterday for Summer Bridge."
On Friday, police turned away staff and students who showed up for that program.
"They're telling me to go home. I came to make sure the kids don't come in, that's what I was told, but they're not letting me in," said school assistant Nora Linares.
Area residents said the blast was terrifying.
"The room I was in shook. The whole room shook," said Marble Hill resident Emily Langer. "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."
"It was like an earthquake. The floor was shaking, we all ran, we was so scared," 11th grader John Torres told CBS2's Ilana Gold.
Neighbors said they thought the worst when they heard the loud boom.
"We thought it was a terrorist attack," said Jackie Morris. "We really did not know what it was."
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