First-responders rushing against the fleeing crowds initially believed a jetliner from nearby San Francisco International Airport had gone down in the San Bruno neighborhood or that terrorists had struck - or both.
"I was concerned about a secondary explosion. I didn't know what we had," San Bruno Fire Capt. Bill Forester recalled Tuesday. "I breathed a sigh of relief when I saw it was not an airplane - there would have been more victims."
What turned out to be a gas line rupture last Thursday fueled a roaring blaze so intense that it cracked windshields of the closest fire engines and sent four firefighters to the hospital for smoke inhalation. The blast also broke a water main, leaving all hydrants in the area dry.
"It was a sinking feeling," Forester said. "We needed massive water for this thing - and we discovered there was no water."
While residents helped crews drag hoses from nearly 4,000 feet away to supply water, other first-responders, including South San Francisco police Lt. Ron Carlino, pushed into smoke-filled homes to check for survivors. Searing heat prevented them from getting too close to the heart of the fire.
"We were left helpless," Carlino said. "The wall of fire was incredibly, intensely hot. We were helpless knowing there were people we couldn't get to."
Many of the 400 police officers and firefighters who responded to the explosion, which claimed at least four lives and destroyed nearly 40 homes, acted despite the dangers: Some were fighting for a neighborhood they grew up in, the homes of friends and streets where children played.
"I saw smoke and flames, and I knew I had to go," said South San Francisco police Detective Ken Chetcuti, who grew up in the area. "I was thinking to myself that I knew a lot of people in that neighborhood."
Authorities said Tuesday that three people remained missing, all of whom lived at the same address. About 10 investigators were working to locate them, said San Bruno police Chief Neil Talford.
"It's forensic work with the coroner's office to identify any remains, as well as locating any individuals who may still be out there," he said.
The San Mateo County coroner identified Elizabeth Torres, 81, who lived just yards from the source of the explosion, as one of the people killed. Her two daughters and son-in-law were seriously injured and remained hospitalized with burn injuries, according to the woman's grandson, Frank Torres.
Also killed were Jessica Morales, 20, Jacqueline Greig, 44, and her daughter, Janessa, 13.
Meanwhile, federal investigators said they wrapped up their on-site probe and had moved on to interviewing witnesses of the blast.
Christopher Hart, vice chairman of the National Transportation Safety Board, said investigators still don't know what caused the Pacific Gas & Electric Co. gas transmission line to rupture and blow a section of 30-inch pipe out of the ground.
The NTSB removed parts of that pipe and shipped it to a lab where it will be tested. Officials could not say how long it will take the agency to release the test results, which could show if corrosion or some other factor caused the rupture.
Investigators were also constructing a timeline of the incident and were looking into why it took PG&E crews nearly two hours to shut off the gas, allowing it to fuel the flames. The company has said it took time to manually turn the valves for the pipes.
At a town hall meeting Monday night, Democratic Rep. Jackie Speier questioned if automatic shutoff valves were necessary.
Hart noted that the NTSB had made recommendations to PG&E following a 1981 gas line rupture in downtown San Francisco, and that investigators would check if the company had complied with those recommendations, which included adequate training for emergencies.
"We will be looking at how quickly and effectively they responded, and that's one of the reasons the timeline is so important to us," Hart said.
In Washington, D.C., Transportation Secretary Ray LaHood announced the allocation of $5.9 million Tuesday for 17 research projects to improve pipeline safety.
The awards will pay for the development of research projects that address the detection, prevention and characterization of pipeline leaks and pipeline construction quality, as well as alternative fuels transportation.