CNN contacted Sidney Alford, founder of Alford Technologies, a British company specializing in explosives, to test detonate a device closely replicating the pressure cooker bombs allegedly deposited last Saturday night in Manhattan by Ahmad Khan Rahami.
In describing this demonstration, CNN has taken care not to provide details of bomb components that are not already widely known.
Law enforcement sources tell CNN that according to an initial assessment of the pressure cooker devices, they contained aluminum powder, ammonium nitrate and HMTD.
Alford told CNN that the correct preparation of these components would likely have resulted in a "much more violent explosive," pound for pound, than the devices in Boston.
The test detonation, which CNN filmed on Thursday in the United Kingdom, produced a powerful fireball scattering shrapnel and small fragments of the pressure cooker over a wide area.
Alford placed two boards of plywood in the vicinity of the pressure cooker bomb he detonated to illustrate the force of the blast. The explosion and shrapnel pierced the wood.
"Anything that goes through both these pieces of wood has great lethal potential," Alford told CNN.
"This was a vicious device. I would not have been surprised if 20 people had been killed in New York," Alford said after the demonstration.
Placement of bomb may have saved lives
According to U.S. court documents, the device that went off in Chelsea appears to have been placed in a heavy metal dumpster. Alford believes that this may have prevented significant loss of life by containing the force of the blast.
Both the Boston and New York devices involved pressure cookers and shrapnel, but the Boston devices contained black powder extracted from fireworks, a much weaker form of explosive. The pipe bomb device set off in a trash can along the route of a U.S. Marines charity race in New Jersey on Saturday morning also included black powder, and thus would have been also significantly less potent than the Chelsea devices, according to Alford.
Authorities believe Rahami was linked to that device, and to the backpack left near a train station in Elizabeth, New Jersey. Authorities have not yet revealed what was inside the devices recovered in that backpack.
Put simply, according to U.S. court documents, the Chelsea bombs contained high explosive, while the Boston devices did not.
This is where the science comes in. Alford explains that high explosive is a term used for explosive that produces what bomb chemists call "detonation."
"This is where the decomposition takes place in the very short time that it takes a shock-wave to pass through the explosive, generating a mass of hot gas that expands at a velocity greatly exceeding the speed of sound," he said.
The Boston devices and the New Jersey pipe bomb did not explode at anything near this velocity because they did not contain high explosive.
Looking for clues in shrapnel, metal pieces
Alford said that if the Chelsea bomb "detonated" it would likely have blown the pressure cooker into many very small fragments, which would have acted as shrapnel and penetrated victims' bodies. If the bomber had not got the recipe right then it is possible the device did not achieve "detonation."
Alford said a tell-tale sign of an explosion short of detonation would be if the pressure cookers had exploded only into larger pieces. But according to court documents, investigators discovered fragmentation from the Chelsea bomb as far as 650 feet away and the force of the blast shattered windows 400 feet away and three stories high. The force of the blast propelled the metal dumpster 120 feet across the street. Twenty-nine people were injured.
As law enforcement cordoned off the area, investigators found an unexploded pressure cooker four blocks away.
The combination of aluminum powder and ammonium nitrate produces an explosive substance similar in composition to Tannerite, a patented substance used on some shooting ranges in the United States to create spectacular explosions when a target is hit by a bullet.
Both aluminum powder and ammonium nitrate can be purchased for this specific purpose in the United States.
"It's made very easy for the amateur operator, rather than someone trained, because you can buy kits that contain the two components. That makes bomb-making much easier as far as the main charge is concerned because you need to know no chemistry at all," Alford told CNN.
Rifle bullets produce enough energy to detonate the explosive mixture. But in the Chelsea pressure cooker devices, a small amount of HMTD could have been used to set off a "main charge" of aluminum powder-ammoniun nitrate mixture, according to Alford.
HMTD has been used to build detonators in multiple al Qaeda plots against the West. A case in point was the July 7, 2005, London bombings. The British al Qaeda operatives who plotted to explode 10 transatlantic passenger aircraft in 2006 also planned to build detonators containing HMTD, according to court records.
Overseas training to make a device deadlier?
HMTD has many similarities to TATP, another high explosive used by terrorist groups in plots against the West.
The chemicals necessary to make both of them can be easily purchased in the United States and instructions for how to make them can easily be found online. HMTD was used by militaries in the years after World War I, but was subsequently abandoned in favor of more stable compounds.
Once dry, the reactiveness of HMTD and TATP means there is a risk of accidental detonation if they are jostled or exposed to high temperatures.
The use of HMTD in the Chelsea devices may point to overseas training. It is tricky to make for individuals without a chemistry background and is difficult to perfect without experimentation. There have been very few Islamist terrorist plots in the West since 9/11 in which plotters have managed to make HMTD or TATP without overseas training.
One of the few exceptions was Andrew Ibrahim, a British self-radicalized extremist arrested in 2008 who made HMTD to insert inside a suicide bomb vest without ever apparently getting training overseas. According to Alford, it is possible to make HMTD just by downloading instructions from the internet, so it does not necessarily indicate overseas training.
One key question is, did Rahami receive bomb-making training from a militant group while he was overseas?
Investigators are probing lengthy stays
by Rahami in Afghanistan and Pakistan between 2011 and 2014. His travel pattern has similarities to that of Times Square bomber Faisal Shazad, an American recruited by the Pakistani Taliban, who trained him in explosives in the tribal areas of Pakistan. Shahzad, who like Rahami was not on the radar screen of U.S. counterterrorism agencies, attempted to set off a car bomb in Times Square in May 2010 half a year after receiving training.