People leaped from third-floor windows and hung from ledges by their arms as thick, black smoke billowed from a large apartment complex late Wednesday. At least 16 people were reported injured.

Residents heard a loud boom and smelled smoke at the Country Club Villas in unincorporated Arapahoe County. The call of an apartment fire on the 8800 block of East Florida Avenue came in at 11:02 p.m.

In all, 16 people were reported injured in the two-alarm blaze, six were taken by ambulance, with two of them suffering serious injuries requiring immediate care, said Fire Chief Jerry Rhodes of the Cunningham Fire Protection District.

The full tally of people injured may rise because some residents were driving themselves to area hospitals, Rhodes said.

Two people at the scene said they heard a boom and smelled smoke. They saw a woman leap from a second-story window. Fire alarms rang out as about 50 people gathered in the street near the burning building.

Seven minutes after the first 911 call was made, the first Cunningham crew of firemen arrived at the large X-shaped complex with four wings.

“They could see a glow of a fire in a second-floor apartment,” Rhodes said. “People were jumping. Others were hanging off of windows.”

That indicated to firefighters that hallways were likely filled with heavy smoke and were impassable, he said.

The on-scene commander almost immediately ordered a second alarm and classified the fire as a mass casualty incident. Firefighters with the Denver and Aurora fire departments were called in as well as six ambulances. The commander split assignments, ordering ladder rescues and two immediate fire-suppression attacks, Rhodes said.

“We were able to rescue many who were hanging by their arms from window ledges,” Rhodes said.

Other people were leaning the top half of their bodies out of windows, he said.

One fire hose was taken up to the second floor onto the hallway, where there was a tremendous amount of heat. Firefighters sprayed down the hallway, opening up an escape route, while other firefighters sprayed from outside onto the fire.

The blaze was extinguished within 15 minutes, but it took another two hours to snuff out all the hot spots in the apartment where the fire broke out and in the hallways.

Around 150 people were evacuated from their apartments. Many were taken to shelters arranged by the Red Cross.

“We have yet to have an opportunity to speak with the people who were in the unit where the fire started,” Rhodes said. “We’re still trying to sort out people.”

It’s unclear how many people were in the apartment and how seriously they were injured. Some were hurt while falling from windows, and the first priority was saving lives and getting people emergency care, he said.

Investigators will track down the occupants of the room and try to determine the cause of the fire, Rhodes said.

The Arapahoe County Sheriff’s Office is also involved in the investigation into the cause.

Rhodes said that the fire would not have spread so quickly if the older apartment complex had a sprinkling system like newer apartment buildings.

“We would not have had this many people affected if the building had sprinklers,” he said.

He added that the last time he was involved in a similar case in which people were leaping from windows was about 20 years ago at the same apartments. People were injured at that time as well, but there were no fatalities.

About 50 people were later allowed to return to their apartments.

Kirk Mitchell: 303-954-1206, denverpost.com/coldcases or twitter.com/kirkmitchell