Rescue teams search for people trapped in a Texas hotel and investigate an apparent gas explosion that injured at least 21 people.

(CNN) — Investigators are working to determine the cause of an apparent gas explosion at a downtown Fort Worth, Texas, hotel that injured at least 21 people as crews searched the building for anyone trapped inside, officials said.

An explosion at the Sandman Signature Fort Worth downtown hotel on Monday sent thick plumes of smoke billowing over downtown skyscrapers and strewn debris onto roads as firefighters rushed to treat the injured and rescue those trapped in basements.

Firefighters believe the explosion was caused by “some type of gas explosion,” but they are still working with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives to determine the cause, a department spokeswoman said Monday. Fort Worth firefighter Craig Trojac at a news conference.

Trojacek previously said there were reports that the explosion may have occurred at a restaurant where construction was taking place, but “we’re not 100% sure where it actually started.”

There is currently no indication that the incident is criminal, a federal law enforcement source told CNN.

The nearly 104-year-old building, which officials said was recently renovated, is located in a busy downtown area near the Fort Worth Convention Center and City Hall.

“It was chaos,” Trojecek said, describing crews arriving at the fire and blowing out windows and walls.

A witness reported seeing people leaving the hotel with bloodied faces and people on stretchers after the explosion, the Fort Worth Star-Telegram reported.

Of the 21 injured, one person suffered serious injuries while four others were in critical condition, according to a spokeswoman for MedStar, which provides emergency medical services and operates an ambulance in the area. While others were said to have minor injuries.

Three employees of Sandman Hotel’s Museum restaurant were injured in the blast. The museum was closed at the time and no customers were dining when the explosion occurred, the restaurant shared in a Facebook update.

The hotel was open and operating when the explosion occurred, with 26 rooms occupied at the time, according to Trojesek, who noted that the hotel was undergoing renovations at the time.

Crews plan to continue canvassing the hotel throughout the night to make sure no one is trapped in the building, firefighters said Monday night.

The Sandman Hotel said in a statement that it is working with first responders and hopes to help those injured.

“Emergency services are at the scene and we are working closely with the authorities to understand the origin of the incident and the extent of the damage caused,” the statement read. “The safety and well-being of our team members and guests is our priority. “We are working with those injured to provide them with full support at this time.”

Texas mayor: It’s “heartbreaking for downtown Fort Worth.”

Workers inspect the damage near the Sandman Signature Hotel after Monday’s explosion. (Credit: Julio Cortez/AP)

After the blast, debris covered the street outside the hotel, where parts of the building’s facade appeared to have been torn off by the blast.

Fort Worth Mayor Matty Parker said the explosion was “heartbreaking for downtown Fort Worth.”

“It’s a beautiful building that was recently renovated and opened, so we’ll be working with the owners of that building as quickly as possible,” Parker said at Monday’s news conference. “Our hearts and prayers really go out to the victims at area hospitals.”

The mayor said she was not in the area at the time, but nearby government buildings were evacuated.

City Hall staff heard the explosion when it happened, and many said it felt like an earthquake, the mayor said.

Across the street from the hotel, Thompson’s Bookstore and Speakeasy suffered significant damage from the explosion.

“Everyone in the building is OK at the time, but it will be a while before we understand the full extent of the damage,” Clinton Bone of Engmar Retail Group told CNN in a statement.

He added that the bookstore has suffered significant damage in the vicinity of the hotel and in the speakeasy area, which is underground.

The store shared on Facebook that the location will be temporarily closed until further notice. “The safety and well-being of our customers and staff is our top priority and we appreciate your understanding during this difficult time,” the Facebook post read.

Texas Governor Greg Abbott said in a statement that state officials are in contact with Fort Worth authorities and are ready to send additional help if needed.

“The State of Texas is in close contact with Mayor Parker, Sheriff Weyborn and other local partners and first responders in Fort Worth to assist with emergency response efforts following the tragic explosion at the Sandman Hotel,” the governor said.

— CNN’s Sarah Dewberry, Amanda Jackson, Josh Campbell and Steve Almasi contributed to this report.