FORT WORTH, Texas (AP) — An explosion at a historic Texas hotel in Fort Worth on Monday blew out windows, littered downtown streets with large sections of debris from the building and injured 21 people, including one who was in critical condition, authorities said.

The blast flung doors and entire sections of wall onto the road in front of the 20-story hotel, where authorities said rescue crews found several people trapped in the basement. Fifteen people were taken to hospitals, including six whose conditions were described as “semi-critical” by MedStar, which provides ambulances and emergency medical services in Fort Worth.

Authorities did not indicate anyone was missing but fire officials said late Monday that they would continue searching the building.

More than two dozen rooms were occupied at the Sandman Signature Hotel at the time, officials said. Authorities said they believe a gas leak caused the explosion — which happened in the middle of the afternoon at the start of the workweek — and said the hotel had been undergoing construction.

“There was debris. There was insulation. There was office furniture,” Charlie Collier, 31, told The Associated Press. He was working nearby when he said he saw a large flash and what sounded like thunder.

“Everything that was in the first couple floors of the building was blown out all over the street,” he said.

The Sandman Signature Hotel is in a busy area of downtown about one 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 coated the street outside the hotel. Authorities urged people to avoid the area.

Craig Trojacek, a spokesman for the Fort Worth Fire Department, said a restaurant in the building had been under construction but said it was not definitive that is where the blast occurred.

Rebecca Martinez was among those in nearby buildings who recalled hearing a loud crack and seeing a wall of dust and debris sweep through the city streets. Stepping outside to see what happened, she came upon a man and a woman leaned up against a fire hydrant.

“The man was all bloody, his face was all bloody,” Martinez said. “Then I started smelling natural gas, real intense and I thought, ‘I might need to get away from here.’”

Moments later, she said, authorities evacuated her building and some of the surrounding neighborhood.

Paula Snider, a delivery driver for UPS, was doing a pickup nearby when she said she heard a large boom and saw a puff of black smoke. A large piece of metal grating landed under her truck and another dropped nearby.

“I jumped out and took off running,” she said.

A grey haze covered normally busy streets of downtown Fort Worth as firefighters walked through layers of debris. Remnants of the building lay scattered across the street and over parked vehicles, and gaping holes could be seen on the ground.

Trojacek said investigators were working with federal investigators to determine the cause of the blast. The scene had left rescue teams unable to immediately reach some parts of the building.

“We had reports of people trapped down in the basement, and because of the explosion that took place, some of those access areas were either covered up or it didn’t feel safe at that point to get people down into,” Trojacek said.

Technicians from Atmos Energy, a Dallas-based natural gas distributor, were examining the blast site Monday. A spokesperson for the Railroad Commission of Texas, the state’s oil and gas regulator, said an agency inspector was also on the scene and working with local authorities.

Northland Properties Co., the Canadian company that owns the hotel, said in a statement that it was working with officials to determine how the explosion occurred and how much damage it caused.

“The safety and well-being of our team members and guests is our priority,” the company said. “We are working with those who have been injured to fully support them at this time.”

According to the hotel website, the Sandman Signature Fort Worth Downtown Hotel has 245 rooms and was built in 1920 as the “Waggoner Building,” named after cattle rancher and oilman William Thomas Waggoner. The building has been listed on the National Register of Historic Places since 1979.

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Associated Press reporters Jake Bleiberg in Dallas and Jim Vertuno and Acacia Coronado in Austin, Texas, contributed to this report.