A construction worker lit a match, sparking an explosion that ripped open a section of a Marble Hill high school Thursday night.

Mayor Bill de Blasio said Friday that the construction worker had been checking whether a gas line was safe. Three construction workers had been working in a science lab on the sixth floor of John F. Kennedy High School on Terrace View Avenue when the explosion happened at around 8 p.m.

All three construction workers remain in critical condition at Jacobi Medical Center. A custodian who saw the workers after the blast said they were injured badly. "Shirts all burned up, their faces, hair - patches burned off," said Nelson Blasini. 

The workers were all employed by the same Queens-based contractor. They were building a new science lab for the upcoming school year.

The damage appears to be heaviest in three classrooms on the sixth floor of the building. The Department of Buildings is assessing the structure.

The building houses seven schools and serves thousands of students. It may not be open by the time school starts on Sept. 9. Some students will be transferred to nearby In-Tech Academy.

Rep. Eliot Engel (D-Bronx) expressed concern about the blast. "The fact that we are only a few short weeks away from students returning to the campus for the school year makes the incident even more terrifying," he said. 

A summer program was being held at the school, but that program is now expected to be held elsewhere.