DALLAS (AP) — An explosion engulfed a Dallas apartment building in a massive and deadly blaze Thursday while firefighters were on their way to a reported gas leak, and three people were killed, officials said.

Dallas Fire-Rescue spokesperson Jason Evans announced the fatalities at an evening news conference. Authorities would not provide an immediate number of how many missing people they were looking for hours after dozens of firefighters had brought the flames under control.

At least four injured people went to hospitals, according to Dallas-Fire Rescue.

It was unclear how many residents lived in the two-story building in the Oak Cliff neighborhood of Dallas. Some residents' friends and relatives worried as they tried unsuccessfully to reach each loved ones.

"There have been fatalities at this point," Dallas Fire-Rescue Deputy Chief Mark Berry said at a news conference, adding the mission has changed from rescue to recovery. "Let us work through the recovery phase and get a total number."

Dozens of firefighters searched through the smoldering rubble of the building on the outskirts of downtown Dallas, even as colleagues continued to drench the blackened debris.

Berry said firefighters were responding to a call of a gas leak when an explosion happened.

"We had the cavalry coming," Berry said. "But the explosion had already taken place."

Atmos Energy, a natural gas provider, said in a statement they were told by fire officials that a construction crew unrelated to the company had damaged a pipeline near the site of the fire.

Kacee Proctor, a resident of the apartment building, said her mother had smelled gas inside a day earlier, but Proctor didn't think much of it at the time.

She wasn't home during the blast and was devastated that her cat, Shirley, was stuck inside.

"I've been sitting over there crying for several hours. I don't know what to do. This is all I have right here," Proctor said, gesturing to the clothes she was wearing.

She spent the afternoon chatting with neighbors who had evacuated, including a girl who was home babysitting her little sister and carried both the child and their dog to safety.

Natural gas service to the area remained shut off, and company officials were working with investigators on-site, the company said.

Authorities set up a family reunification center at a nearby high school. Several hours after the blaze, Frances Rizo was still trying to find her friend who lived in the building.

"She's not answering her phone," Rizo said.

Firefighters rushed to the scene as flames and black smoke billowed into the sky. Some trained their hoses on piles of smoking debris while others removed lumber and other burned wreckage to look for anyone trapped underneath. Little more than a blackened shell of the original building remained.

"The fire is contained, but our members are still working on the scene to do primary searches," said Dallas Fire-Rescue Assistant Chief James Russ.

Julie Jensen said she was at home less than a block from the burning 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.

Sal De La Rosa was at work at a nearby auto repair shop when "all of a sudden we just heard and felt this huge boom."

"We felt where the building kind of shook a little bit," Del La Rosa said.

He said a co-worker went outside and saw thick, black smoke rising into the air.

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This story has been updated to correct the spelling of Frances Rizo's last name in one instance. It is Rizo, not Rizzo.

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Associated Press journalists Jim Vertuno in Austin, Texas, Russ Bynum in Savannah, Georgia, and Hannah Schoenbaum in Salt Lake City contributed.