New York City firefighters battled a blaze at a Bronx high school possibly caused by a gas explosion Thursday night that injured three people, officials said.

"It is not under control; it is an ongoing scene," an FDNY spokesman said late Thursday night, adding that the department had deployed 31 units and 138 people to the John F. Kennedy High School campus, in the Marble Hill section.

Mayor Bill de Blasio, who traveled to the site, told reporters: "What we see here at JFK High is a really shocking scene of the sixth floor having experienced a tremendous explosion."

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He added, "Residents here in Marble Hill -- they felt a blast, they felt a building shake."

Three construction workers were seriously injured with burns, and are being treated at Jacobi Medical Center in the Bronx, the mayor said.

"What caused the explosion? We do not know that yet," he said, adding the investigation will continue.

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Several months ago, the School Construction Authority hired a private contractor to improve the school's sixth floor science laboratory, which has gas burners, officials said.

The Department of Buildings is checking the school's structural integrity to determine whether the 4,000 students who attend the eight schools in the building can start the autumn semester at their campus or will have to be relocated.

Fire Commissioner Dan Nigro said the blast occurred shortly after 8 p.m. "These victims had gotten out of the area of the explosion," he said. "Our folks found them. One was more seriously injured than the other two."

Nigro said the contractors did not mention smelling gas.

Con Edison spokesman Alfonso Quiroz said the utility could not confirm there had been a gas explosion at the school, but noted there was "major structural damage to the fourth, fifth and sixth floors." The gas has been shut off to the building, he said.

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The contractors were working for Positive Associates General Construction, which was hired by the School Construction Authority, a police spokesman said.

No further information was immediately available.

The school at 99 Terrace View Ave. is in the Bronx, but the Marble Hill neighborhood is part of the borough of Manhattan.