ERIE, Colo. — 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.

Erie Middle School was closed for summer break at the time of the explosion.

Investigators said they haven't determined what caused the blast, but John Poynton, spokesman for the St. Vrain Valley School District, said initial reports indicate a water heater may have exploded in a part of the school that is being renovated.

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

Officials did not release the identities or conditions of the three victims, but McConnell said they suffered fall- and burn-related injuries.

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.