SUPERIOR, Colo. -- A fire that gutted three townhomes in Superior last week started near the air conditioner behind the unit where a 13-year-old girl was baby-sitting her younger sister.

Mike Tombolato, fire chief of the Rocky Mountain Fire District, said he has sent that air conditioner to a lab so it could be examined closely to see if there was a mechanical problem or perhaps a mouse or bird's nest that sparked the fire.

Tombolato also said the fire started outside of Unit 285, spread along the adjacent decks and went up the outside walls and into the upper floors.

"It was obvious that the fire got large in a hurry," said Tombolato.

Heraa Hashmi was watching her baby sister when she smelled smoke and sensed that something was wrong. She walked up from the basement to the main floor and saw flames burning in the kitchen and near the entrance.

"There were flames everywhere. The sofa was melting," Hashmi told 7NEWS.

She ran back down the basement, grabbed her sister and called 911.

"I am 13 years old and I am home alone with my baby sister and my house is on fire," she told the dispatcher.

The dispatcher told her, "I need you to take your baby sister and get out of the house."

Hashmi said there was a problem when she tried to get out of the house with her sister.

"The curtains on the back door were also burning, but the door wasn’t, so I grabbed a jug of water and threw it on the flames," she said. "The fire went out a little, so I just slid the back door open, held my sister and crawled out into the back yard."

"It would have been quite a story for a fire fighter to be doing it in full gear, let alone a 13-year-old girl," said Tombolato. "To be able to think so clearly and decisively to pull that off (is amazing)."

Tombolato said if Hashmi would have hesitated, even just for 15 seconds, both she and her sister could be dead.

Since the fire, dozens of people have donated cash and household goods to help the families who've been burned out.

The lead pastor at Rock Creek Church said they're now asking local businesses to step forward.

Three townhomes at the Rock Creek development were destroyed and three others were heavily damaged.

The Colorado Bureau of Investigation and the Bureau of Alcohol, Tobacco and Tobacco, Firearms and Explosives have agents who are assisting in the investigation.