Mahwah, NJ – MAHWAH — Officials at the Bergen County fire academy are probing the cause of a propane gas explosion that occurred Friday while instructors were setting up a training exercise.

Only one of the instructors, Steven Alvarez, was inside the 20,000-square-foot building during the blast, authorities said.

Alvarez was preparing training exercises a floor below the explosion, said James Mordaga, director of the Bergen County Law and Public Safety Institute.

“It knocked him to the ground, but he was in full protective gear,” Mordaga said. “He was just shaken up.”

The concrete building is fireproof, Mordaga noted.

Officials believe the explosion occurred when a pilot light in one of the training rooms failed to ignite the propane gas used to simulate fires.

When it finally did ignite, there was enough propane gas in the room to cause an explosion, said Chief Fire Instructor Lawrence Rauch, who also heads the Upper Saddle River Fire Department.

Officials present several scenarios for firefighters in the large building — among them, kitchen and couch fires. The system that’s used, he said, is similar in some ways to a barbeque grill and became a popular alternative to the use of hay in training exercises several years ago.

The building remains temporarily closed while officials check the other units in the structure, Mordaga said.