ERIE, Colo. (CBS4)- Three people have been injured from fire and falling debris after an explosion at a middle school.

The explosion happened at Erie Middle School just after noon on Wednesday. The force of the explosion caused part of the roof to collapse.

“All of a sudden this loud boom explosion came through my back and I quickly turned around and saw a pretty good sized piece of equipment flying through the air, 50 to 60 feet in the air,” said John B. Vanderpool, who lives near the school.

Two construction workers are listed in critical condition and the third is in good condition and was expected to be released from the hospital Wednesday evening. All three suffered burns and other injuries. No students were in the school at the time.

The explosion and roof collapse happened at the school’s gymnasium. Investigators still don’t know what caused the explosion.

Firefighters were on the scene within minutes of the explosion. They were having a hard time entering the building because of the air quality.

The school is undergoing a major reconstruction project.

Mike Meade and his neighbors say they heard a deafening explosion.

“It was one of those shocking moments, like, ‘That was an explosion — where?’ ” Meade said.

“I kind of thought, ‘They must have dropped something.’ I didn’t realize how serious it was,” neighbor Ozzie Mutz said.