(CNN) -- Last Thursday was back-to-school night in many of the communities that dot the peninsula below San Francisco, California.
Off-duty firefighters and police officers who went to schools with their children and families soon learned something horrible had happened in a San Bruno residential neighborhood.
Speculation ran from an airplane crash to an earthquake to terrorism. It was only later that they learned there was a gas transmission line explosion.
South San Francisco firefighter Anthony Ottoboni was at his son's baseball practice when he got the news.
"My wife looked right at me and said, 'Go.' "
Capt. Bill Forester of the San Bruno Fire Department was on duty.
"We looked out the back door and saw a fireball," he said.
John Priolo, also with the fire department, recalls rushing to work and seeing the "walking wounded" arrive at the station.
On Tuesday, first-responders gathered near the fire site to talk about the heroics of others, including citizens, last week. They spoke of the strong fraternity of the 400 firefighters, paramedics and police officers who thronged to the neighborhood.
They mentioned how their training prepared them, but there are some things training can't foresee.
"Somebody said it was like they took a Saturn V rocket and tipped it upside down at the blastoff," Forester said. The high-pitched roar of gas sounded like a jet engine, he added.
Chaos greeted them as they moved into the area.
Many people were running away. Bystanders were taking photos. Always, there was the wall of fire and oppressive heat.
"I realized immediately how overwhelmed we were going to be," said Sgt. Mike Guldner of the San Bruno Police Department.
Guldner and some civilians helped a seriously injured man. "Thank God to the people and citizens of San Bruno who stepped up to provide assistance to me," Guldner said.
Firefighters quickly learned the explosion had taken out the water mains.
"It's a sinking feel to say the least because you count on that water being there," Forester said. "If there was ever a time you need it, it was there."
But firefighters improvised, stretching lines from another water grid.
The responders, many of whom knew people who lived in the Crestmoor Canyon neighborhood, went to smoke-filled houses, making sure everyone got out. They were assisted by several residents.
"We got as close to the flame as we possibly can without getting ourselves burned," said Officer Ken Chetcuti of the South San Francisco police.
Lt. Ron Carlino, also of the South Francisco police, recalled the dazed homeowners who grabbed a few belongings.
"Some people wanted to grab just anything," he said. "They felt like they couldn't leave empty-handed."
Four people died in the fire, and three are still missing, officials said Tuesday. Thirty-seven homes were destroyed.
Homeowners have since returned to Crestmoor Canyon and are trying to patch their lives back together. But the people of San Bruno have changed, according to Scott Rogge, a local police officer.
"We had everyday people running into houses, helping us," he said.
Since then, the community has received and given donations, opened a shelter, provided food, clothing and more.
"People that normally you see, maybe wave -- they are exchanging e-mails, phone numbers, hugs and tears," Rogge said.
"Those are the true heroes, the people of the community. ... We do this every day. We've chosen to do this. But those people stepped up."