Nine people were hospitalized - with two of the injured in serious condition, and one in critical condition.

Authorities say witnesses smelled something like paint burning before the explosion occurred, which officials chalked up to a natural gas leak.

"There is a smell of gas here in downtown. We're not sure if the smell of gas was caused from the explosion and the fire itself or if that's what caused the explosion. But that's what we're looking at," said Fort Worth Fire Department spox, Craig Trojacek.

MedStar said nine people were hospitalized. Four of those patients were sent to John Peter Smith Hospital in Fort Worth.

Fort Worth Fire said they have firefighters inside the building searching for survivors who may be trapped. Trojacek said they'll assess the stability of the building after search and rescue is complete. One person reportedly remains unaccounted for.

From Texas Sky Ranger, glass and part of the building's facade from at least the first two floors on multiple sides of the building were visible along 8th Street and in a parking lot on the building's west side. -NBC DFW

One witness working at a nearby coffee shop told NBC 5 that he heard the explosion and saw debris and white smoke coming from the building. Another witness, a valet named Adam, said he was walking in the area when another valet told him to avoid 8th street.