FORT WORTH, Texas >> Fire investigators said Tuesday they have still not determined what caused an explosion at a historic Texas hotel that plunged two stories of debris into the basement, blew out windows and doors onto the streets of downtown Fort Worth and left more than 20 people injured, including one person who remained in critical condition.

Officials have said that a gas leak was likely involved in Monday’s blast, which blew doors and sections of wall into the road in front of the 20-story downtown Sandman Signature Hotel, where authorities said rescuers found several people trapped in the basement. But the local fire chief acknowledged Tuesday that they remain uncertain about what led to the blast.

“Obviously there was gas — natural gas — involved,” Fort Worth Fire Chief Jim Davis said Tuesday at a news conference. “We do not know if gas caused the explosion or the explosion caused the gas problem. So that’s part of the investigation.”

The fire department is working with state and federal investigators and utility workers to determine what caused the explosion that Davis said piled the building’s basement with debris. Firefighters with dogs searched the rubble overnight, but the chief said Tuesday that there’s no indication anyone is missing.

Fifteen people who were hurt in the mid-afternoon blast were taken to hospitals, including one who was in critical condition, according to Fort Worth’s emergency medical services provider. Officials said more than two dozen rooms were occupied at the hotel when the explosion happened.

An official with one city hospital said Tuesday that four of the seven patients they received were treated and released. Two were admitted and one was sent to a Dallas hospital with major burns. Davis said some people suffered concussions from the blast.

“There was debris. There was insulation. There was office furniture,” said Charlie Collier, who was working nearby and said he saw a large flash and what sounded like thunder.

The hotel is in a busy area of downtown about a block from the Fort Worth Convention Center. Footage from news helicopters showed firefighters picking their way through the piles of drywall, shattered glass and mangled metal that lay scattered across the street and over parked vehicles. Gaping holes were visible in the ground.

Fire officials said the chaotic scene made it impossible for rescuers to reach parts of the building immediately after the blast.

“Two stories of debris collapsed into the basement, where several of the victims were found,” Davis said.

In photos that the fire department posted on social media, firefighters could be seen lifting a woman out of what appeared to be the hotel’s lower level. Her eyes appeared to be closed, and her face and hair were speckled with dirt and debris.

Along with firefighters, that blast site was being examined by technicians from Atmos Energy, a Dallas-based natural gas distributor and an inspector with Texas’ oil and gas regulator, officials said. Northland Properties Co., the Canadian company that owns the hotel, said in a statement that it is also cooperating to determine the cause of the explosion.

The building that is now the Sandman Signature Fort Worth Downtown Hotel was built in 1920 and has been listed on the National Register of Historic Places since 1979. Craig Trojacek, a fire department spokesperson, said its restaurant has been under construction, but it’s not clear whether that’s where the blast occurred.

The owner of the Japanese restaurant, Musume, said in a statement Tuesday that it was closed during the blast and that three employees are among those sent to hospitals.