CRYSTAL LAKE – Two firefighters were sent to the hospital for heat exhaustion Wednesday after a roof fire at Crystal Lake Central High School, fire and police officials said.

Crystal Lake Fire Rescue Deputy Chief Christopher Olsen said crews were dispatched about 3:40 p.m. to the high school, 45 W. Franklin St., for reports of smoke coming from the roof.

It’s not yet clear what was burning, Olsen said, but the first units to arrive on the scene saw thick, black smoke coming from the school’s roof. And while the fire happened in an area of the school that’s under construction, the cause of it has not been determined.

“The fire was contained to the roof area,” Olsen said. “Right now, we don’t know what caused the fire. We’re still investigating that.”

The two firefighters were taken to Centegra Hospital – McHenry, Olsen said later Wednesday night in a news release. Both firefighters were from Crystal Lake Fire Rescue Department, he said.

Crystal Lake Police Deputy Chief Tom Kotlowski said maintenance workers in the building were evacuated.

Several fire trucks filled up Walkup Avenue in front of the school, and residents watched from surrounding streets as crews worked.

Olsen said a mutual aid box alarm was upgraded to the second alarm level, bringing in additional fire units from around McHenry County. Units from Algonquin, Lake in the Hills, Barrington-Countryside, Carpentersville, Cary, East Dundee, Fox River Grove, Huntley, Marengo, McHenry Township, Nunda Rural, Wauconda, West Dundee, Wonder Lake and Woodstock fire departments assisted.

With temperature in the 80s, Olsen said the department rotated crews often.

“We have a rehab area where the firefighters are being evaluated medically and able to get something to eat, something to drink,” Olsen said.

He said the Salvation Army and McHenry County Emergency Management Agency assisted with rehabilitation.

Crystal Lake-based Community High School District 155 officials released the following statement Wednesday evening:

“The CL Fire department responded to a structure fire around 3:40 this afternoon at Crystal Lake Central High School. Everyone was evacuated safely and accounted for. There is no structural damage. The fire is out and firefighters are assessing the damage. Activities will resume tomorrow as scheduled.”