The official investigation of the gas explosion that killed a coffee shop owner and badly damaged several buildings in Durham last week is still underway. One thing is clear though, the toll for the city's firefighters was high: nine of them were among the 25 people injured.
Jerry Knapp, who trains firefighters how to respond to gas leaks, says responding to reports of gas leaks is a much bigger part of the job for firefighters than most people know.
"Here in Rockland County we did about 700 gas leaks last year, over 26 fire departments," he said.
And even though few of the thousands of leaks reported annually around the country result in injuries or destruction, the risks of facing an invisible threat that can instantly destroy an entire city block are extraordinary.
When leaking gas builds up inside a structure where there are potential ignition sources like pilot lights and electrical devices, it becomes so dangerous that Knapp refers to it as "improvised explosive building."
"The point I like to make to the guys is that it goes from zero-to-60 in a microsecond," he said. "You go from joe jireman, where everything is fine, to a position where you're not sure whether you're dead or alive."
Because reports of leaks are so common, one thing firefighters are up against when they respond to one is complacency.
"I used to parachute, and I have a picture of my reserve parachute, and I show it to them," Knapp said. "And I tell the guys that if my main parachute didn't work, I had a plan B. When you go to a gas leak, you don't have a plan B. Our turnout gear isn't rated for explosions, so we don't have any protection from that, and if that building takes off, it's just good luck, and the Grace of God that will determine if you survive or not. So your plan A has got to be right, you can't get complacent. You just can't."
Knapp says each department will typically have its own standard procedures − which Durham officials said firefighters followed here.
Those, he said, start dictating things even before firefighters arrive on scene.
"You know, we always get an address. Obviously, if it's a gas leak, we don't want to pull up before that building or that house. If it does go bad, we're in what I call the 'kill box.'"
The kill box is the name he uses with firefighters he's training to emphasize the risks for anyone close enough to a potential explosion to be hit by a deadly blast wave, fire, or debris.
The firefighters would typically stop a few hundred feet away from a reported leak, and start assessing the situation, taking into account the details provided by whoever reported the leak.
"As you approach the scene, you want to look for third party damage. Is there a backhoe that hit the line? Were they excavating, digging a hole for something?" Knapp said. "You're also listening. If it's a high pressure leak, you'll hear it. And if we can, we ask our dispatcher to tell the caller to come outside the building to meet us. So we want to determine, first, if we can find an obvious cause for it, and then work down from there. But the key is, I train guys to assume the building is going to explode until you prove otherwise."
From the minute they get the call, the firefighters are constantly trying to piece together information as they get more, he said. And it can initially be scarce.
"A real problem is when you get there, you can't see it," Knapp said. "If there's a car accident, you can see it. If there's a building on fire, you can see it. So when I train guys, I always say we're chasing the invisible man at night. You have to use your instruments to find it. And it takes time to do that."
But they might not have time. The gas might be reaching an ignition source, just seconds or minutes after they arrive.
The instrument he mentioned essentially sniffs the air for gas, then says whether it's there, and if so whether it seems to be in dangerous amounts.
As firefighters work their way toward the reported leak site, he said, they can use it to sample the air at storm sewers and the vents in manhole covers and inside buildings to try to get a sense of what the gas is doing.
But it's not precise, in part because firefighters using them can't sample everywhere, and there might be dense pockets of gas where they aren't testing.
And they continue to gather any other information they can, from residents, police officers who might have arrived before firefighters, or gas company workers − who usually are called immediately so they can help assess the threat and shut down any leaking lines.
"So we'll work our way in and ask questions of the people along the way," Knapp said. "Where do they smell it? How bad is it? Those kinds of things, until we sort it out. And it's really a detective game.
A potentially deadly detective game, with a ticking clock you can't see and clues that can be hard to read.
For example, even when a gas line seems to be broken only outside a building − as apparently was the case in Durham − there are several ways it can seep inside to create that potential bomb.
"We call it migrating gas," Knapp said. "And that's the problem, it will migrate underground. It takes the path of least resistance. So it may go back into the buildings by the sewer lines. It may go back to a crack in the foundation."
And potential ignition sources are numerous, and not always obvious.
A hot water heater or stove may have a pilot light on. And even if it's not, the electronic ignition for them can spark an explosion.
Even shutting off electrical power to potential ignition sources could trigger the startup of emergency generators or battery-powered emergency lights that then could light off an explosion.
"And with solar power, there's almost, like, a secondary electrical system in the building, that you really can't shut off unless you cover the solar panels with a dark tarp or something," Knapp said.
Natural gas is lighter than air, so a clean break outside could spew it harmlessly upward, he said. But a force, from say, a backhoe, that only partly breaks a pipeline outside a building, might also break it under or inside the building. So some gas could be flowing inside.
Sometimes − as happened in Durham − the information they gather quickly tells firefighters they need to evacuate the site.
"Our goal at a gas leak is life safety. So if we can get the civilians out as quickly as we can, and minimize our time in the kill box, that's our goal," he said.
Knapp knows as well as anyone the dangers of being in that box, because of a leak he investigated in 2012.
A contractor hit a gas main; the firefighters had evacuated much of the neighborhood, but as they did, gas from the break was migrating through the soil, where it was being held under a layer of frozen ground − until it got under a house and found an ignition source, maybe a gas-powered water heater.
"We were at the last building and then the house exploded, and I remember feeling the over pressure from the blast," he said. "And it was like I was being hugged instead of being tackled. I was being lifted up, I remember seeing my feet off the ground. And then next thing I remember is, I'm at the curb, you know, bleeding, burned and trying to figure out if I'm alive or not.
They were just 10 feet from the house when it exploded, and were tossed about 30 feet from where they had been standing. Somehow, both survived. Knapp's injuries included broken bones in his face, burns and bruises.
"I got off easy," he said. "My captain was hurt worse than I was. He was out of work 13 months and had severe injuries."
Better technology may help firefighters keep themselves and civilians safe. Knapp has been urging the adoption of a laser-based device that can detect gas from a distance, or even through the window of a building.
But, he said that the job will always be dangerous, even with better equipment and even when you follow every procedure protocols.
"As firefighters, we like to think we take manageable risks," he said. "We don't trade firefighters lives for civilian lives. But we take manageable risks. And we do that with, you know, structure fires and high-angle rescue and everything else we do. So we try to manage that risk appropriately."
Manageable risks, though, are still risks. And firefighters take them every time they arrive at a reported gas leak.