Why the Chelsea bomb didn’t kill anyone, according to bomb experts

There’s a lot authorities still don’t know about the bombings that shook Manhattan and New Jersey this weekend. Did the sole suspect, Ahmad Khan Rahami, have concrete ties to the Islamic State or al-Qaeda? Was he part of a larger terror cell? And perhaps most pressingly, could other attacks be imminent?

But there are two things that forensics experts inside and outside of the government agree on: The bombs were relatively sophisticated, and if not for one key mistake, the attacks would have been much, much deadlier.

Rahami was arrested Monday morning in New Jersey after a brief gun battle with the police and remains under heavy guard at a local hospital. The FBI is closely examining his cellphone, computer, and personal belongings for any signs that he was in contact with ISIS, al-Qaeda, or other terror groups. (A notebook recovered by investigators contained handwritten references to both Boston Marathon bombers and the American-born al Qaeda propagandist Anwar al-Awlaki.)

Authorities are also trying to determine if Rahami, a naturalized US citizen who was born in Afghanistan, may have been radicalized during any of his multiple visits to Pakistan over the past decade. FBI spokesperson Kelly Langmesser declined to comment on the investigation.

Forensic experts, meanwhile, are zooming in on how the bombs were triggered and where they were placed. What they’re finding could help us better understand these attacks — and the ones potentially still to come.

How a dumpster saved lives

Rahami may have thought he had good reasons for allegedly stuffing the Chelsea bomb inside a large metallic dumpster. It meant the bomb was hidden from police and passersby. It also meant that the dumpster itself could have been torn into pieces of jagged white-hot shrapnel in the ensuing explosion, dramatically increasing the amount of damage to anyone unlucky enough to be standing nearby.

That’s not what happened. Instead, the dumpster appears to have contained much of the blast, ensuring that a bomb capable of potentially killing dozens of people ultimately wounded 29 but caused no fatalities.

John Goodpaster, a forensic chemist at Indiana University–Purdue University Indianapolis, said that placing a bomb into a container where it is surrounded on all sides except for one — think of a box with its top removed — leads to so-called “focusing.” That, he explained, “means the blast is largely contained and nearly all of the explosive force will be directed out of the opening of the container.”

Since the dumpster was open on the top, he added, “the explosive force was directed upward — which is exactly the opposite of what one wants for maximum casualties.”

David Foran, the director of Michigan State University's forensic science program, said the dumpster probably “helped contain the shrapnel,” which means that Rahami may have inadvertently done the exact opposite of what he may have hoped to accomplish in the first place. The dumpster didn't create new shrapnel; it kept what was already packed into the bomb from causing even more casualties.

These bombs were more advanced than the Boston ones

When Tamerlan and Dzhokhar Tsarnaev set off pressure cooker bombs near the finish line of the Boston Marathon in 2013, killing three people and wounding 260 others, they detonated the explosives with remote controls fashioned from model car parts. That meant they had to be standing physically close to the bombs, at least theoretically making it easier to have identified and caught them.

The Chelsea and New Jersey bombs, by contrast, were triggered with cellphones, which would means they could have been set off from almost anywhere. A would-be terrorist could have been much farther away from the blast sites than Rahami was when he was arrested.

That difference, said Adam Hall, a forensic chemist at Northeastern University who formerly worked with the Massachusetts State Police, means that “in some ways, in comparing the two devices, the one in Manhattan was a higher level of sophistication.”

Goodpaster said most of the IEDs seen to date in the US used fuses, with the next level up involving timers and the more advanced iterations using remote controls or cellphones. Those types of bombs, Goodpaster said, were commonplace on the battlefields of Iraq and Afghanistan but “extremely rare in the US.”