(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.
Thirteen other firefighters are being treated at local hospitals, said fire Capt. Ruy Lozano. One of them is in critical condition with burns in an intensive care unit.
Lozano said the five-alarm fire started at a restaurant next to 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. State and federal officials will begin their investigation Saturday.
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.
"We will improve, we will get better, we will learn from this," he said, "and we will keep on keeping on."
CNN's Ryan Rios and Sara Ganim contributed to this report.