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DES MOINES, Iowa -Des Moines Fire Department Spokesman Brian O’Keefe told Channel 13 that investigators don’t believe a fire that broke out in a century-old downtown building was intentionally set. But he added they still don’t know what caused Saturday morning’s fire at the Equitable Building.

SATURDAY’S STORY: the Equitable Building, which suffered the fire, was built 90 years ago and had been undergoing renovations. The late developer Bob Knapp once had plans to turn the 19 floor structure into high-end condominiums. But money problems eventually doomed the project, as well as Knapp’s eventual admission he tried to cut costs by improperly removing asbestos. Knapp got sentenced to federal prison and ultimately killed himself following his release.

ORIGINAL STORY: Five firefighters were injured while responding to a fire at the Equitable Building in downtown Des Moines early Saturday.

Fire crews responded to the Equitable Building, 604 Locust Street, shortly after midnight Saturday. A skywalk security officer reported hearing an explosion and strange noises coming from inside the building.

PHOTO GALLERY: Fire Damages Equitable Building in Downtown Des Moines

The Des Moines Register reports that two firefighters were injured by falling debris, one fell off an 8-foot ladder and two others were treated for heat related injuries. Their injuries are not life-threatening.

Firefighters were able to contain the fire to a crawl space. Heavy heat and water damage was reported on the second story of the building. Smoke damage was reported in the upper floors of the building. Smoke also affected the Hub Tower, Liberty and Ruan buildings through the Skywalk system.

Crews remain on scene as of 10:30 a.m. to watch for hot spots and to investigate the cause of the fire. About 50 firefighters responded to the scene, the Register reports.

Locust Street is currently closed between 6th Avenue and Eighth Street as crews remain on scene. Officials hope to reopen the road to traffic in the next few hours.

The American Red Cross responded to the scene to provide food and water to first responders.