Nearly 100 firefighters battled a large fire that erupted on Thursday at an apartment complex in Dallas, where debris from one collapsed building lay heaped on the ground while flames and black smoke billowed into the sky.

News video showed dozens of firefighters at the scene. Some had hoses trained on piles of smoking debris while others lifted and moved lumber and other burned wreckage as if searching for people underneath. Two firefighters with hoses towered above the blaze on long ladders, while another sprayed water from the window of an upper floor of a neighboring building.

A natural gas explosion initially caused the fire, the Dallas fire rescue assistant chief, James Russ, said at a televised press conference.

“The fire is contained, but our members are still working on the scene to do primary searches,” Russ said. “We put a drone in the air to canvass the area to see if we see any victims or anything around.”

Four people had been taken to the hospital with injuries, Russ said, though he did not know what condition they were in. It was “unknown how many possible fatalities we have”, he added.

A nearby street was lined with firetrucks, ambulances and police vehicles with their lights flashing.

Julie Jensen said she was at home less than a block from the building when she heard a noise like an explosion that left her ears ringing.

“I was sitting on my couch watching TV – stuff flew off our walls,” Jensen said.

Jensen said she saw rising smoke and neighbors running when she looked out the window. She grabbed her family’s cat and left, finding a nearby parking lot to wait until she knew it was safe to return.

“I am closely monitoring the latest developments regarding the explosion in Oak Cliff,” Dallas’s mayor, Eric Johnson, tweeted. “My prayers are with those injured, their families, and all those who have suffered loss from this tragic event.”

A spokesperson for Dallas fire rescue did not immediately return phone and email messages seeking comment. The Dallas police department referred all questions to fire officials.