FORT WORTH, Texas (AP) — Firefighters scoured the wreckage of a historic Fort Worth hotel on Tuesday as they investigated the cause of a massive explosion that shattered windows, littered the streets with debris and injured 21 people, including one critically.

The Monday blast 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. By evening, officials said everyone seemed to have been accounted for, but firefighters with dogs continued searching the rubble overnight.

The fire department is working with state and federal investigators and utility workers to determine the exact cause of the explosion, though authorities believe a gas leak was to blame. The department said Tuesday that no more victims had been found and that the area around hotel would remain closed.

Fifteen people who were hurt in the mid-afternoon blast were taken to hospitals, including one who was in critical condition and six others whose conditions were described as “semi-critical” by MedStar, which provides ambulances and emergency medical services in Fort Worth. Officials said more than two dozen rooms were occupied at the hotel when the explosion happened.

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“There was debris. There was insulation. There was office furniture,” said Charlie Collier, a 31-year-old who was 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 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. There were gaping holes visible in the ground.

Craig Trojacek, a fire department spokesman, said a restaurant in the building had been under construction but that it hasn’t been determined that the blast occurred there. Authorities have not specified the cause of the suspected gas leak.

Rebecca Martinez was in a nearby building Monday when she recalled hearing a loud crack and seeing a wall of dust and debris sweep through the streets. Stepping outside, she came upon a man and woman leaning 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 UPS driver, 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.

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

“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,” he 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.

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.

“We are working with those who have been injured to fully support them at this time,” the company said.

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.

Its Japanese restaurant, Musume, advertises a private dining room “set within one of the building’s decades-old, original bank vaults.” The restaurant’s owner said in a statement Tuesday that it was closed during the blast and that three employees are among those hospitalized in stable condition.

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