21 injured in explosion at downtown Fort Worth hotel ( www.nbcnews.com )

The explosion at the Sandman Signature Fort Worth Downtown Hotel appears to have been caused by “some type of gas leak," the Fort Worth Fire Department said, noting that the cause had not been confirmed.

Twenty-one people were injured in an apparent explosion at a downtown hotel in Fort Worth, Texas, on Monday afternoon, authorities said.

Firefighters initially received calls about a structure fire shortly after 3:30 p.m. at the Sandman Signature Fort Worth Downtown Hotel, and then quickly after received "multiple calls that were coming in that area there was a type of explosion," said Craig Trojacek, a spokesperson for the Fort Worth Fire Department.

Aerial footage from NBC Dallas-Fort Worth showed what appeared to be sections of wall and parts of window frames blown out of the first and second floors of the building. Debris, including insulation and parts of windows, was scattered on the sidewalk and road.

The number of injuries was revised up to 21 from the initial 11 later in the night.