A restaurant collapsed after an explosion on Thursday, trapping at least one person inside and injuring at least one other person, police said.

The injured person was found outside the building, but it wasn't clear whether the person was inside the Branch Inn at the time of the explosion, said Police Sgt. James Ruggieri. There was no immediate word on the person's condition.

Authorities are investigating what might have triggered the explosion.

"It's flat to the ground," said Rick Beatty, a delivery driver for the nearby Flower and Herb Co.

Flower company owner Mardi Clay said the explosion was so loud she thought a car had smashed into her shop. "It rattled the whole block" and sent smoke billowing, she said.

Pueblo, a city of about 106,000, is 100 miles south of Denver.