While details about the chain of events that led to the horrific East Harlem gas explosion that killed seven and injured dozens of others remain murky, there's one thing that quickly became clear: The blast was caused by a natural gas leak, and at least some of the devastation might have been preventable.

Some nearby residents have told reporters that the smell of gas was noticed yesterday and, in some cases, has been lingering for weeks. But Bob McGee, a spokesperson for Con Ed, said the first call reporting the odor only came in yesterday morning, around 20 minutes before the explosion.

"Gas would have had to accumulate in a certain area where it wasn't venting, and saturate the air at a 5 to 15 percent level," he said. "This really underscores the importance that whenever people smell gas, they should call us immediately." Oddly, McGee said, gas is only combustible at a saturation point between 5 and 15 percent—no less, no more.

"You can actually have an area that's saturated that's not in an explosive range, that then comes into an explosive range as the gas itself is being dissipated," he said.

McGee said that anyone who smells gas should call Con Ed immediately, but not before getting a safe distance away. Placing a cell phone call—or flipping a light switch, or turning on a television—could all trigger an explosion. Anything that involves an ignition is a potential hazard—which is why Con Ed crews bang on the door instead of ringing the bell when responding to a call.

In the meantime, the company will begin the long, painstaking process toward figuring out the precise cause of yesterday's blast. Cameras will be sent through the gas mains, and metallurgists will disassemble pipes in search for answers. McGee said that two surveys were completed on February 10 and 28 to check for weaknesses in the neighborhood's pipes—potentially caused by the year's abnormally rough winter—though nothing turned up. He added that of the 30,000 gas-odor calls the company fields each year, very few result in catastrophe.

Natural gas itself is odorless—a chemical called mercaptan is added to ensure that people are aware of its presence in the air. In June, Harlem residents reported that upgrades to a gas pipeline were resulting in the smell of fresh-baked cinnamon buns.

McGee said it's too soon venture any sort of guess at what went wrong. "Until such time as we can figure out where exactly this happened and what caused it, we really can't say," he said. Could the snow have played a part? "There's nothing right now that would lead one to think that," he said.