Cody Duty / Houston Chronicle via AP A firefighter is wheeled to an ambulance after fighting a fire at the Southwest Inn on Friday, in Houston.

State and federal authorities are investigating the cause of a massive five-alarm blaze at a Houston motel Friday afternoon that killed four firefighters and injured 14 more, marking the single deadliest day in the history of the Houston Fire Department, according to officials.

The fire is believed to have sparked at a restaurant adjacent to the Southwest Inn just after 12:07 p.m. Friday afternoon before swelling into a monstrous inferno, engulfing part of the motel in flames and thick plumes of smoke, according to Houston Fire Department spokesman Capt. Ruy Lozano.

First-responders arrived at the scene at 12:11 p.m. to rescue motel guests. But at some point during the blaze’s ferocious tear through the motel, one of the building’s structural components collapsed and the firefighters became trapped beneath the wreckage, according to Houston Fire Chief Terry Garrison.

“They were risking their lives to save our community,” Garrison said at a Friday press conference.

Although authorities have yet to determine the cause of the lethal blaze, Lozano said that the cause of the death for the four late first-responders was the structural collapse.

The victims have been identified as Capt. EMT Matthew Renaud, 35, of Station 51; engineer-operator EMT Robert Bebee, 41, of Station 51; firefighter EMT Robert Garner, 29, of Station 68; and firefighter Anne Sullivan, 24, of Station 68, who graduated from the Houston Fire Department Academy in April, the department said in a news release.

“This was the worst day in the history of the Houston Fire Department with the most lives lost,” said Mayor Annise Parker at a Friday press conference.

A 1929 incident in which three firefighters died after their Engine was broadsided by a train previously marked the deadliest day in the department’s 118-year history, according to a department press release.

One of the 14 injured firefighters was in critical condition Saturday, with three others in stable condition and a fourth awaiting surgery. All others have been released from a local hospital, Lozano said.

The Bhojan Restaurant, an Indian café neighboring the motel, reportedly has a history of citations by city inspectors, including one in March for allegedly failing to clean grease traps on restaurant grounds, according to The Houston Chronicle. However, authorities have not determined if Friday’s deadly inferno was initially triggered by a grease fire.

The first five months of 2013 have been particularly lethal for Texas firefighters, with 13 on-duty fatalities reported, including at least five deaths stemming from the horrific fertilizer plant explosion in the city of West on April 17.

Officials announced last month that ammonium nitrate caused that deadly accident, which left at least 15 people dead and more than 160 injured.

NBC News' Becky Bratu contributed to this report.

Flags were flying at half-staff Saturday in Houston as the city mourned the loss of four firefighters who were killed trying to save the community. Diana Alvear reports.

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