Firefighters with the Pittsburgh Bureau of Fire responded to a two-alarm structure fire in the 30 block of Mount Oliver Street just after 8 p.m.

When crews arrived on scene they found heavy flames at the rear of one of the structures. The fire quickly grew to four alarms. 

There were two separate collapses at the rear of the building and a mayday was called out at one point until all of the firefighters were safely accounted for. 

Four firefighters were injured. One suffered a minor burn injury, while another received a laceration. Two others sustained shoulder injuries. All four were transported by EMS to the hospital for treatment and evaluation.

All of the residents of the three affected structures exited safely, but they will be assisted by the Red Cross with finding temporary accommodations.

The fire was contained mainly to the center building, but the two adjoining buildings received water and smoke damage.

The fire was declared under control at 10:22 p.m. The Fire Investigation Unit is investigating the cause and exact origin of the fire.

The Department of Permits, Licenses and Inspections will be called in to assess the integrity of the structures.