A preliminary investigation indicates that a cigarette not properly put out sparked a five-alarm fire that forced residents of a Robbinsdale apartment building to run to safety early Friday, authorities said.

Firefighters from 13 departments responded to the blaze at the Beach South at the Lake Apartments on the 4200 block of 46th Avenue N. The first crews arrived on the scene at 1:28 a.m., and it took 110 firefighters to knock down the fire. Some were still on the scene eight hours later, said Robbinsdale Fire Marshal Greg Bodin.

No residents were injured, but three firefighters suffered second-degree burns and were taken to North Memorial Medical Center in Robbinsdale for treatment. They were later released, Bodin said.

As the fire quickly turned into an inferno, smoke alarms did sound to help wake up residents.

And police went door to door to make sure everybody got out. Everybody was accounted for, Bodin said.

The building was not equipped with a sprinkler system, but it did meet code because it was built before such systems were required, Bodin said.

Much of the building just off Hwy. 100 near Twin Lake was gutted. Twelve units were declared uninhabitable, Bodin said.

The Red Cross and the Salvation Army were providing assistance to between 45 and 50 people who were forced out of the building.

One woman who lived in the building for nearly 30 years told a Red Cross volunteer that she heard what she thought was hail. Moments later, she said, she saw smoke and sparks and was told by police to evacuate immediately.

Most residents were going to stay with friends and family members, but some were being put up in hotels, said Red Cross spokeswoman Lynette Nyman.

This was the third night this week that crews from west-metro fire departments have battled blazes. Many of them come from on-call departments, so they are feeling a bit weary after putting in long hours. “Fatigue is setting in,” Bodin said.

A local bakery donated doughnuts to feed the firefighters.