Rescue workers transport one of the victims of an explosion at Erie Middle School to a waiting medical helicopter on Wednesday, Aug. 1, 2012. ( JEREMY PAPASSO )

ERIE -- Two construction workers and a school district employee were injured Wednesday -- two critically -- following an explosion in Erie Middle School's gymnasium that shook nearby homes and caused part of the building's roof to collapse.

No students or teachers were in the building at the time of the explosion, which occurred in a part of the school that currently is being renovated, officials said.

While investigators said they have not determined a cause, John Poynton, spokesman for the St. Vrain Valley School District, said initial reports indicate a water heater may have exploded.

Emergency dispatchers began receiving calls around 12:13 p.m. Wednesday about an explosion at the Erie school, 650 Main St., according

A firefighter exits Erie Middle School on Wednesday, Aug. 1, 2012, after searching for people trapped inside following an explosion and partial roof collapse. ( JEREMY PAPASSO )
to Laura McConnell, a spokeswoman with the Mountain View Fire Protection District.

When emergency crews arrived, they found the explosion had caused part of the gym's roof to collapse. Three people suffered fall- and burn-related injuries, McConnell said, with two being transported to local hospitals by ambulance and one victim airlifted from the scene.

By Wednesday evening, officials had not released the identities of the three victims, although McConnell confirmed that two remained in critical condition at Front Range hospitals and the third was in the process of being released at around 6:30 p.m.

Earlier, dispatchers described at least two of the victims as having second-degree burns. McConnell could not confirm the severity of the injuries, but said it was likely the person who was airlifted was taken to a specialized burn unit.

While school was not in session Wednesday, McConnell said there were some school officials present who were overseeing construction inside the building. Although officials initially indicated that all three victims were contractors, Poynton confirmed Wednesday evening that one of them was a district employee.

The explosion could be heard at neighboring homes and caused several neighbors to run outside to see what was going on.

"I was just watching TV when I heard this big old boom and the pictures fell off the wall," said Lisa Armijo, who lives a few doors down from Erie Middle School. "I just grabbed my kids and ran outside. It was pretty scary."

Another neighbor, Mike Meade, said, "I knew it wasn't a kid jumping around or something falling over. You could tell right away it was a pretty big explosion."

Klipp Architecture and Adolfson & Peterson Construction have been removing the school's second floor this summer, as the district expands the school's capacity from 532 students to 950 students.

Of the school's 115,000 square feet, 50,000 square feet are being renovated, while 25,000 square feet are being demolished and replaced, according to officials. Although the second floor is being removed, new classrooms will be added on the northwestern and eastern sides of the school, so it will have 26 classrooms, up from 20.

Construction began in November, while school was in session. Temporary classrooms are being used to house students as the remodeling work is done. The entire project was scheduled to be finished for the fall of 2013.

Classes are scheduled to start at Erie Middle School on Aug. 16.

The Erie Police Department, Lafayette Fire Department and the Colorado State Patrol also responded to assist Mountain View Fire and the Boulder County Sheriff's Office at the scene Wednesday.

Camera Staff Writer Amy Bounds, Colorado Hometown Weekly Staff Writer Doug Pike and Times-Call Staff Writer Victoria Camron contributed to this report.