ERIE, Colo. (AP) — Two construction workers and a school district employee were injured Wednesday when an explosion caused a roof to partially collapse at a middle school east of Boulder.

Preliminary information indicates that the explosion happened as a plumbing contractor was lighting a hot water heater in the mechanical room above the Erie Middle School gymnasium, firefighters said. Witnesses told investigators it was the first time the natural gas had been turned on following renovations at the school.

The school was closed for summer break at the time of the explosion.

Laura McConnell, a spokeswoman with the Mountain View Fire Protection District, told the Daily Camera (http://bit.ly/R9j4J6 ) that when firefighters arrived, they found that the explosion had caused part of the gym's roof to collapse.

McConnell said the victims suffered fall- and burn-related injuries. One was listed in critical condition, one was listed in serious condition, and the third was treated at a hospital and released.

The loud explosion caused residents to run outside to see what was going on in the neighborhood about 14 miles northeast of Boulder.

"I was just watching TV when I heard this big old boom and the pictures fell off the wall," said Lisa Armijo, who lives near the school. "I just grabbed my kids and ran outside. It was pretty scary."

Another neighbor, Mike Meade, said, "You could tell right away it was a pretty big explosion."

Students are scheduled to return to the school Aug. 16.