SAN BRUNO, California -- U.S. federal investigators are examining whether Pacific Gas & Electric workers at a Northern California control center followed proper emergency procedures after a gas transmission line exploded into inferno that caused at least four deaths and destroyed nearly 40 homes in San Bruno.

National Transportation Safety Board vice chairman Christopher Hart said Tuesday that constructing a timeline of how PG&E crews based at a pipeline-monitoring terminal reacted to the blast would be important to determining why it took the utility two hours and 46 minutes to cut the gas that fueled Thursday's devastating blaze.

"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.

The utility has said the pipeline, built in 1956, had to be shut down manually because it was not equipped with automatic shut-off valves newer lines have. At a town hall meeting on Monday night, PG&E vice president Geisha Williams told residents there was a delay in stopping the flow of gas because it wasn't safe for workers to get near the explosion site while the blaze raged.

Hart said during Tuesday's media briefing that the two shut-off valves for the ruptured section of pipe were located 1 and 1 1/2-miles away from the blowout.

The NTSB made recommendations to PG&E for improving its emergency procedures following a 1981 gas line rupture in downtown San Francisco, Hart said. Investigators plan to conduct interviews with workers at the control center to see if workers observed the protocols, he said.

"Investigators are looking in extensive detail how that system worked," Hart said.

Meanwhile, state and federal lawmakers have called on PG&E to upgrade any other pipelines in heavily populated areas that now can only be closed down by hand.

California Assemblyman Jerry Hill, a Democrat whose district includes San Bruno, said Tuesday he was drafting a bill that would require the utility to install automatic and remote shut-off valves to minimize future damage.

"Since only seven percent of the nation's gas lines are classified as a 'high consequence area' like the San Bruno line that exploded, the least we can do is focus resources on these gas lines within California," Hill said.

The NTSB's Hart said investigators have completed their onsite site investigation and shipped a 28-foot-long section of 30-inch pipe that blew out of the ground to a Washington lab for forensic tests that could reveal if the explosion was caused by a small leak or a "catastrophic failure."

In Washington, D.C., Transportation Secretary Ray LaHood on Tuesday 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.

Also Tuesday, police officers, firefighters and paramedics who were among the first to reach the blast recounted the horrifying scene they encountered. Rushing against the fleeing crowds, they initially believed a jetliner from nearby San Francisco International Airport had gone down in the 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. "I breathed a sigh of relief when I saw it was not an airplane -- there would have been more victims."

It took almost a half-hour to determine that the massive fireball consuming a San Francisco suburb wasn't a plane crash.

What turned out to be a gas line rupture last Thursday fueled a roaring blaze so intense 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 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.