At least 21 people were injured in an explosion that was most likely caused by a gas leak and substantially damaged a hotel in downtown Fort Worth on Monday afternoon, the authorities said.

One person was in critical condition and two were in serious condition, Craig Trojacek, a spokesman for the Fort Worth Fire Department, said at a news conference. Eight others had minor injuries, he added. The authorities learned about the additional injuries later, including one person who went to a hospital on their own.

Earlier, the authorities had said that one person was missing, but they noted later that the person had been found.

Initially, the authorities said that it was unclear what caused the explosion at the Sandman Signature hotel, which occurred about 3:30 p.m., though Mr. Trojacek said that there was “a smell of gas in the area.”

“We were getting reports that it had started in the restaurant,” Mr. Trojacek said, adding that a restaurant at the hotel was under construction. “We’re not 100 percent sure that that’s where it actually started at this point.”

Later, the Fort Worth Fire Department said on social media that the explosion had most “likely” been caused by a gas leak, but that officials were waiting to confirm that.

A spokeswoman for Atmos Energy, which provides natural gas to North Texas, said the company was looking into the explosion.

Three of those injured were employees at Musume, a contemporary Asian restaurant located inside the Sandman, Josh Babb, the co-founder of the restaurant said on Instagram. “All of us at Musume are devastated,” he said, noting that the restaurant was closed at the time and that there were no customers inside. The three employees were receiving medical treatment and were in stable condition, Mr. Babb said.

Video footage and images from the scene showed substantial damage to the ground floor of the hotel, a new facility housed in a historic century-old building, and debris littered across the street.

Christian Alvarez, 25, who works at the Pink Cobra, a tattoo parlor two blocks from the hotel, said he felt the shop shake on Monday afternoon. Mr. Alvarez said he and two other co-workers walked outside and saw smoke pluming down the street.

“It was pretty gnarly,” Alvarez said.

Kevin Martinez, a manager at a CVS store nearby, said that the windows of the store had reverberated for a couple of seconds after an initial blast. Some of his co-workers assumed the sound was thunder, as a line of storms had moved through earlier in the day, Mr. Martinez said.

“I said, ‘No, no it’s not,’” he added. “I thought it was a bomb.”

After the shaking stopped, Mr. Martinez said, he went outside and saw people running toward the hotel and the smoke.

“To be honest, it was surprising because there wasn’t much panic,” he said. “I thought they’d be running away from it.”

Another CVS employee, Vrisa Verduzco, said she was on her way into work when she saw police cars racing toward Houston Street. Block by block, they erected barricades. It wasn’t long before the dust, smoke and smell of gas were inside her car.

The Fort Worth Fire Department said that a reunification area for those looking for loved ones had been set up in a parking lot a few blocks from the hotel.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said it was assisting the Fort Worth Police and Fire Departments.

Tim O’Hare, the Tarrant County judge, said on social media that he had directed county buildings in downtown Fort Worth to close for the day out of “an abundance of caution.”

Gov. Greg Abbott of Texas said in a statement that he was monitoring the situation, and that additional resources could be deployed immediately.

“I ask all Texans to pray for those who were injured in the explosion, as well as those bravely responding to keep others safe,” he said.

The hotel occupies one of Fort Worth’s most historically significant buildings. Built in 1920, the 20-story W.T. Waggoner Building was added to the National Register of Historic Places in 1979. The register describes it as a “physical manifestation of the crucial role played by Fort Worth in the national oil boom.”

The building housed the offices of several oil exploration firms and a bank over the years, according to the National Register. Its current owner, Northland Properties, purchased the building in 2019. The 245-room Sandman Hotel opened in March 2023, according to The Fort Worth Star-Telegram.

Livia Albeck-Ripkaand Orlando Mayorquin contributed reporting.