Five people were injured and 18 others were displaced in a fire at an apartment complex in Renton early Monday, said Will Aho, deputy chief of operations with Renton Fire.

The blaze at Seventh Street and Shattuck Avenue South was reported around 5:30 a.m. as a heavy fire involving multiple units of an apartment building, according to the Renton Fire Department.

The fire started in a stairwell, making it difficult for people on higher floors to escape from the building. The cause of the fire is still being investigated, Aho said.

Responding firefighters reported flames coming through the roof of the apartment building and quickly called for a second alarm, the fire department said on Twitter.

Of the five people injured, two were taken to Harborview Medical Center in Seattle with serious injuries, Aho said. Two other people were taken to Valley Medical Center with less serious injuries and on-site medics were able to treat another person who did not need to be taken to the hospital, he added.

King County Fire District No. 20 later said on Facebook that three people had been taken to the hospital.

Renton fire said that a King County Metro bus was at the fire scene to keep people who had been displaced from their homes out of the cold. The Red Cross helped find shelter for those displaced by the fire, Aho said.