HOUSTON, Texas (CNN) -- Four Houston firefighters died after a hotel wall collapsed while they were battling a major blaze Friday, the mayor of the Texas city said.
"Today is going to go down ... as the worst day in the history of the Houston Fire Department," said Mayor Annise Parker.
Six other firefighters are being treated at local hospitals, the mayor said. One of them is in critical condition with burns in an intensive care unit.
Parker and Fire Chief Terry Garrison did not mention whether anyone else -- including civilians -- had been hurt or killed in the blaze, though mayor's office spokeswoman Janice Evans said there are others affected.
Fire Captain Ruy Lozano told CNN affiliate KTRK-TV that the five-alarm fire was at the Southwest Inn, in the southwest part of the city near Interstate 69. More than 150 firefighters responded to the blaze, Lozano said.
Some of them then went in "because we thought we had some civilians in the structure," Garrison said.
"Unfortunately, the building had much more fire in it than we originally thought," he added. "The structure collapsed ... and our members who were trying to save lives were trapped."
Fellow firefighters quickly sprang into action, some digging through the rubble to get to their colleagues. Garrison said the death toll could have been much higher if not for their bravery and quick action.
"(There were) dozens and dozens of acts of courage that took place in the seconds when that wall came in," he said.
The firefighters killed are Matthew Renaud, 35, who joined the fire department in October 2001; 41-year-old Robert Bebee, who started in August 2001; 29-year-old Robert Garner, part of the department since October 2010; and Anne Sullivan, a 24-year-old who graduated from the Houston Fire Department Academy in April.
Authorities haven't given a cause of the fire.
Garrison said his firefighters are "taking care of each other, embracing each other," and taking solace in the support they'll get from the community.
Beyond getting over the loss emotionally, the fire chief promised that his department -- the third largest in the country -- would learn from what happened.