HIGHLANDS RANCH, Colo. -- Nearly two dozen fire crews are battling a two-alarm blaze at an apartment complex at C-470 and Quebec Street in Highlands Ranch.

No residents were hurt but one firefighter suffered heat stress and was treated at the scene, said Littleton Fire Chief Jay Ruoff.

No one was home when the fire started inside one of the buildings at the Palomino Park Condominium Complex, firefighters said.

Firefighters received the call at about 2:10 p.m. and arrived at 6098 Blue Ridge Drive shortly thereafter. They saw heavy fire on the second floor and tried to tackle it from the inside.

However, the blaze was burning so intensely that firefighters were forced to back out and had to wait until more crews arrived.

Eights apartment units are in the condo building that is on fire. At least four of those units are uninhabitable, the fire chief said.

Firefighters have 24 units on scene so crews can rotate in and out as not to tire or overheat themselves, Ruoff said. With temperatures near 90 degrees, and firefighters wearing equipment that weighs 60 to 70 pounds, firefighters can get very hot and tire quickly, he said.

It's not known yet what started the fire, but neighbors were talking about a propane grill on the second floor balcony where the fire may have started.

"I'm just thankful that they got out of the house, since they were sleeping when it started," said Debbie Gary, of her husband and daughter.

Their home has water damage, and they will have to stay with family.

The Red Cross is helping the 21 people displaced by the blaze.