Investigators are still trying to figure out what caused last night's explosion at a high school in Marble Hill that seriously burned three construction workers.

The blast at John F. Kennedy High School happened just after 8 p.m. startling neighbors.

"I was in bed but laying awake, and the floor laterally moved from side to side. It was quite a blast. Scared the hell out of me," said one Marble Hill resident.

"It's like the building had an impact like something hit it hard," said another Marble Hill resident.

Three classrooms were destroyed, and the sixth floor of the eight-story building was heavily damaged.

The fire department says the blast may have been gas related.

Con Edison has not confirmed that it was a gas explosion, and says there was no gas leak.

Mayor Bill de Blasio says construction crews were working on the school's science labs when the explosion occurred.

"Typical School Construction Authority project to improve the science labs – again, being done by a private contractor. I won’t go into a lot of details because we want to do an investigation. I can say this is a contractor that School Construction worked with a lot in the past and had a good reputation," De Blasio said.

"It’s like a typical science lab that has Bunsen burners, so they have gas supplied. No one has mentioned smelling gas. The contractors did not mention smelling gas – no," said Fire Commissioner Daniel Nigro.

The building houses eight schools and more than 4,000 students.

Students showing up for a Summer Bridge program on Friday were sent home.

"I'm like a little creeped out, because what if I was there?" noted one student.

Officials say the building will remain off limits to students until further notice.

"We will never allow students to go back into a building that is not completely safe so right now we are concentrating on fixing those three floors and investigating the cause of this issue," said Schools Chancellor Carmen Fariña.

The chancellor went on to say that she believes repairs can be made so the school can reopen in time for the start of classes on September 9.