FORT COLLINS, Colo. -- Fire heavily damaged two residential buildings in downtown Fort Collins Monday, leaving nearly two dozen residents without a roof over their head.

The blaze began around 3:30 a.m. in an apartment building under construction at Mason and Maple streets, according to Matt Housley, with the Poudre Fire Authority.

It then spread to a nearby condo complex that was already occupied.

“I heard crackling noises,” said Kathleen Ory, a resident who was awakened by the blaze. “I looked around my room and it was all orange.”

Ory told 7NEWS that she looked out the window and saw the building next door engulfed in flames.

“I’ve never seen a fire so out of control,” she said.

The fire then spread to her building.

“I called 911, then grabbed my cat and got out,” Ory said.

Firefighters fought the blaze from inside the second building for about an hour, but as the flames moved into the roof and became more intense, fire crews had to withdraw and commenced fighting the blaze from the outside with ladder trucks.

Poudre Fire Authority Public Information Officer Patrick Love said investigators have not determined the cause of the 3-alarm blaze.

“We’re still dealing with hot spots and are trying to get a handle on the building’s stability,” he said.

When asked if it was flames, embers or heat that ignited material on the second building, Love said, “I can’t answer that.”

“What I do know,” he added, “is that the second building has a fire suppression system, and that system did its job, until it was overrun by the amount of fire.”

Love said that once the fire moved into the roof area and other void areas where there were no sprinklers, the blaze took off.

“It’s really sad,” said Saira Taylor. “I’m glad everyone is O.K., but it’s going to take a while to get back to normal.”

Taylor said she and Ory lost everything in the blaze.

“But it’s all material,” she said. “We still have memories of good times. I’m just thankful that everyone in the building got out safely.” Fire crews have been able to place tarps over some exposed belongings and will try to make arrangements in the coming days to get them back to their owners.

Love said there appears to be heavy fire damage to the fourth floor and roof of the second condo building, and heavy smoke and water damage to the first, second and third floors.