HOUSTON - At least 21 people were injured following an explosion on Monday at a hotel in downtown Fort Worth, Texas, authorities said.

One person was in critical condition and four others were seriously injured, according to Medstar, which provides ambulance and emergency medical services in Fort Worth.

The explosion took place around 3:30 pm local time (2130 GMT) at the 245-room Sandman Signature Fort Worth Downtown Hotel, which was originally built in 1920 as the "Waggoner Building", according to Fort Worth Fire Department officials.

The blast caused heavy debris from the hotel building to scatter throughout the street and forced nearby employees to evacuate.

At least two floors of the 20-storey building's facade were blown out onto the street and into a parking lot, local media reported.

Investigators believe the blast was caused by natural gas, Sara Abel, spokesperson with the Bureau of Alcohol, Tobacco, Firearms and Explosives, told local media.

"There is a smell of gas in the area and there are windows and things that were blown outside of the structure," said Craig Trojacek, a spokesman for the Fort Worth Fire Department. First responders are continuing to search throughout the building.

The hotel opened last May following an extensive reconversion of the Waggoner Building which has been listed on the National Register of Historic Places since 1979, said the hotel's website, noting that "many of the original features" were retained during the reconversion.