“We’ve never seen an improvised thermobaric device before in this country or any country,” says Dave Williams, who conducted the FBI’s on-scene investigations of the World Trade Center, Oklahoma City, Pan Am 103 and Unabomber bombings, among other notorious incidents. Thermobaric refers to a gaseous fuel-air explosion.

“The reason is, it’s very difficult to get the timing down to get an optimum mixture of air and a liquified carbonaceous fuel such as propane, methane, acetylene or natural gas,” Williams told SpyTalk. “He couldn’t have done it the first time and made it work. There had to be a test area.”

Accidental thermobaric explosions are not uncommon—for example, when a house explodes because of a natural gas leak. But IED-makers haven’t tried to stage them deliberately, up to now, Williams says, because too many things have to go right.