Officials in Texas have yet to determine exactly what caused an explosion at a hotel in downtown Fort Worth that injured 21 people, but believe natural gas was involved, Jim Davis, the city’s fire chief, said at a news conference Tuesday evening.

The blast, which occurred at about 3:30 p.m. on Monday at the Sandman Signature hotel, resulted in part of the building collapsing into the basement, leaving one person burned and others with concussions, strains and lacerations from the debris, Chief Davis said. The person who was burned remained in critical condition at Parkland Memorial Hospital in Dallas, he said.

“Obviously there was gas, natural gas, involved — we do not know if gas caused the explosion or the explosion caused the gas problem,” Chief Davis said. “It’s just a matter of the chicken or the egg.”

Earlier Tuesday, at a briefing before the City Council, the Fort Worth police chief, Neil Noakes, said that officials had “absolutely no indication that there was any intent by anyone to cause this explosion.”