An explosion ripped through the floors of a hotel in downtown Fort Worth, Texas on Monday, leaving aout 21 people injured.

According to Fort Worth Fire Department Public Information Officer Craig Trojacek, one of the 21 people wounded in the explosion was in critical condition while four are in serious condition. 14 of the injured were taken to local hospitals.

Explaining the incident, Trojacek said that a call came in from Sandman Signature Fort Worth Downtown Hotel at around 3:32 p.m. That call was followed by multiple calls reporting an explosion. Per Trojacek’s statement, rescue teams were unable to reach some parts of the building at first.

“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.

However, by Monday evening, things had calmed a bit and the fire department said that the explosion did not pose any safety issues to downtown Fort Worth. However, fire crews reportedly continued their search efforts at the explosion scene all through the night in order to make sure no one was trapped in the ruins.

Trojacek further revealed that investigators have reason to believe that the blast was caused by a gas leak. However, the official cause has not been concluded on, as investigators and federal investigators are working together to determine what led to the explosion.

Photos of the scene show blown out windows, scattered debris and piles of rubles. Per authorities, construction had been going on in the building when the blast occurred.

Witnesses in the area at the time of the explosion reportedly heard a loud noise. Mike Vanca, one of the hotel’s guests, who was in a nearby office away from the hotel at the time, said that he felt the shockwave from the blast from where he was.

According to him, the hotel building “shook like someone slapped the side of it with your hand.”

Some others, like an employee at a nearby restaurant, said they smelt natural gas.