THE BRONX - Emergency crews responded to an explosion at John F. Kennedy High School in Marble Hill.

Witnesses say they heard a loud boom at around 8 p.m. Thursday. The blast rattled windows of nearby residents' homes.

Officials say the explosion may have been caused by gas in a sixth-floor science lab. The damage sustained by the school appears to be contained to three classrooms.

Officials say three contractors who suffered serious burns were transported to Jacobi Medical Center -- two  of them in serious condition and the other in critical condition. The private contractors were working the science lab as part of a project that began when school ended for the summer.

Police blocked off the area surrounding the school as part of their investigation. Gas services to the school were shut off.

Mayor Bill de Blasio held a news conference at the scene Thursday night. He lauded the response of all emergency services.

FDNY Commissioner Daniel Nigro says engineers from the Department of Buildings were on scene to check the structural integrity of the school.