Houston (CNN) -- When Joshua Gandara heard his cousin, Houston firefighter Robert Bebee, had died suddenly, he instinctively knew why.
"I knew (he was) saving somebody," Gandara said. "That's him. ... He always put people first, before him. Anybody's needs before his needs."
Relatives of the 41-year-old Bebee -- including his parents, sister and nieces -- are not alone in their mourning. Four Houston firefighters, total, died when a wall collapsed during a major blaze Friday at a hotel.
There was Matthew Renaud, a 35-year-old who worked out of five stations in the 12 years he was with the Houston Fire Department. Robert Garner, 29, had been at Fire Station 68 since joining the department in October 2010. The youngest victim was Anne Sullivan, a 24-year-old who graduated from the city's firefighter academy in April.
"She was very passionate, very strong, (very) beautiful, and she was very dedicated to her career," Sullivan's friend and fellow firefighter Ryche Guerrero said. "She inspired me."
The five-alarm blaze began in the Bhojan Vegetarian Indian Cuisine restaurant, where "the majority of the fire was ... in the front," according to fire Capt. Ruy Lozano. It ended up spreading to the adjacent Southwest Inn, which is in the southwest part of Houston along the Southwest Freeway.
Recalled Guerrero, "I'm just amazed at how big the fire was and how quickly it spread, how the smoke was just billowing in different directions."
City fire Chief Terry Garrison explained Friday that some of the more than 150 firefighters responding to the blaze went inside the structure "because we thought we had some civilians in the structure."
As Lozano explained, it was during normal business hours and there was reason to believe people we're inside.
"There was every indication to think that there was a life to be saved," he said.
But when they got in, the first responders found "the building had much more fire in it than we originally thought," according to Garrison.
Parts of the structure collapsed, killing three firefighters at the scene, and leading to the death of the other at a hospital.
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," the chief said.
Thirteen other firefighters were hurt in the incident, five of whom were still hospitalized Saturday afternoon. That figure includes one who Lozano said is in "extremely critical" condition and one who had surgery Saturday and faces "probably more surgeries to come."
The fire captain told CNN there are "a multitude of situations going on" related to their ailments, from heat exhaustion to injuries to their legs and more.
Given the total number of casualties, Houston Mayor Annise Parker said Friday will "go down ... as the worst day in the history of the Houston Fire Department."
Members of the department, which is the third-biggest in the nation, are leaning on each other to get over the loss of their colleagues.
"We're one of the biggest families you'll ever meet," Lozano said. "We're very close. ... We take pride in having a little city attitude in a big city."
The building that went up in flames had been inspected, with follow-up visits, within the year, said Lozano, who didn't offer any details on what they found.
Authorities have not yet determined the cause of the blaze. Members of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were at the site on Saturday, as were members of the State Fire Marshal's office, the Houston Fire Department's homicide division and the Texas Rangers.
"By bringing in these outside agencies and treating it like a crime scene, and there's no reason to believe that it is, we're able to maintain that sterile environment in efforts not only to determine the cause but also to hopefully learn from it so, hopefully, it never happens again," Lozano said.
A funeral service for the four firefighters killed is scheduled for Wednesday at Reliant Stadium, home of the NFL's Houston Texans. In addition to trying to help the families of those killed, the fire department is also trying to take care of its own.
"The firefighters are having to look to each other, look to their families, look to their faith and look to the firefighter support network to get through it," an emotional Lozano said.
"... They also think about when they get home and their kids ask, 'What happened?' And how do you tell them? So it's a difficult time."
Assistant State Fire Marshal Kelly Kistner told CNN that 18 Texas firefighters have died on duty so far this year, including 10 first responders killed in an explosion at a fertilizer distribution facility in West, Texas.
To put this number into context, 83 firefighters were killed on the job nationwide all of last year, the U.S. Fire Administration reported.
Already, more Texas firefighters have died in 2013 than over the past five years combined, according to Kistner.
"When we have one line of duty death, that's too many. And we're at 18 this year," Kistner said. "It's a rough year for Texas."
CNN's Sara Ganim reported from Houston, and CNN's Greg Botelho wrote and reported from Atlanta. CNN's Ryan Rios contributed to this report.