An explosion and fire were reported Thursday at an apartment building in Dallas’s Oak Cliff neighborhood, sending about 100 firefighters to the scene in a three-alarm response. Dallas Fire-Rescue said several victims were reported, and at least one person was taken to a hospital for treatment.

The fire was reported around 1:30 p.m. near the corner of East 9th Street and North Patton Avenue. Neighbors said they heard a loud boom and felt their own apartments shake, and people in the area also reported hearing an explosion.

The size of the response showed how seriously crews treated the call, but the cause of the explosion and fire was still unknown Thursday. It was also not yet clear how many people were inside the building when it started, leaving firefighters and investigators with two of the most basic questions still unanswered as the scene remained active.

For residents nearby, the blast was the kind of noise that stops a block in its tracks. For officials, it became a race to check for injuries, search the building and figure out what went wrong in a dense apartment area where one incident can quickly become many.