Several witnesses told 7NEWS that a man carrying what appeared to be a fuel can was seen running from the apartments.

A fire spokesman said injuries ranged from minor to moderate.

The fire was reported just after 11 p.m. at the four-story Fitz Apartments, 1747 N. Peoria St., fire officials said.

Several people were trapped on upper floors and firefighters on ladder trucks rushed to rescue them.

"I've got a lot of rescues going on all around this building," a fire official radioed at one point.

A woman trapped in a fourth-floor bathroom was "screaming for help," a firefighter radioed.

Another fire official said there were people trapped on the backside of the building and firefighters "need water to make a rescue," according to emergency radio traffic.

Fire officials set up a "casualty collection area" near a 7-Eleven on the north side of the apartment complex, according to radio traffic.

By 11:52 p.m., fire officials reported the fire was under control but not fully extinguished.

The apartment complex is located across the street from the University of Colorado's Anschutz Medical Campus.

Investigators on Tuesday will begin work trying to determine what caused the fire.