WASHINGTON — The Boston Marathon bombing marked a watershed moment that counterterrorism officials and specialists have dreaded for years: the use of multiple “improvised explosive devices,’’ or IEDs, to inflict mass casualties on US soil.
Islamic militant groups routinely have used makeshift bombs — detonated nearly simultaneously — to kill civilian bystanders as well as military personnel from Iraq and Israel to Russia and Afghanistan. But until Monday’s explosions at the finish line in Copley Square, such street-level detonations targeting sidewalk or marketplace crowds had not occurred in the United States.
“The sad thing is that finally someone has carried it out in a way that caused deaths and injuries,” said former homeland security secretary Michael Chertoff, who is now a security consultant. “They appear to resemble something seen overseas. A classic method of foreign terrorists carrying out attacks is to have multiple explosions.”
Just two months ago, the White House established an IED task force in the Department of Justice to apply lessons for domestic defense that have been learned from years of dealing with the phenomenon on foreign battlefields.
“The threat from IED use is likely to remain high in the near future, and will continue to evolve in response to our abilities to counter them,” stated a strategy document approved by President Obama on Feb. 26. The intent of the new task force was to improve coordination “to best position the United States to discover plots to use IEDs in the United States, or against US persons abroad.”
Whether Monday’s bomb attack that killed three and wounded more than 170 was the work of foreign terrorists, or a domestic group or individual adopting Mideast terror tactics for their own ends, remains unknown.
Chertoff stressed that “I’d be cautious about looking at it as a particular signature of a particular group.”
Homemade bombs have been a weapon of choice of the Irish Republican Army and antigovernment groups in the United States in past decades. But the dual bombings on Monday more closely resembled the modern playbook of Islamic extremists — both in technique and materials used, specialists said.
Two law enforcement officials said the devices were fashioned out of household pressure cookers, a relatively rudimentary but highly effective design used by insurgents overseas — especially in Afghanistan, where it is a common household item. Reports from medical staff at Boston hospitals who treated victims in the Marathon attack indicate the bombs were packed with BBs, nails, and other projectiles intended to maximize injuries.
Instructions for building a pressure-cooker bomb are contained in Al Qaeda terrorist handbooks and homegrown anarchist publications. One was built for the failed attempt to explode a car in New York’s Times Square in 2010. They also have been used in attacks in Pakistan and India.
The FBI and Department of Homeland Security have been particularly worried about pressure cookers in recent years.
A 2010 warning from the FBI and Department of Homeland Security said “terrorists can exploit the innocuous appearance of easily transportable items such as pressure cookers to conceal IED components. Placed carefully, such devices provide little or no indication of an impending attack.”
As forensics experts scrambled Tuesday to identify the bombs’ makeup for possible clues to who made them, current and former counterterrorism officials said the tactics marked a new chapter in the struggle to secure public places from attack.
“We’ve seen this around the world, but what is so concerning about this is that it hadn’t happened here, not like this,” said a senior homeland security official in Washington.
A recent US intelligence analysis contained examples of domestic groups and individuals, their motivations, and how they might operate now. The review, by the US military’s Northern Command, categorized “single-issue organizations” such as the Oklahoma City bombers in 1995; the so-called Olympic bomber, Eric Rudolph, who committed several bombings between 1996 and 1998; or the “Unabomber,” Ted Kaczynski, who mailed bombs to professors and corporate executives over 20 years.
“These groups often try to challenge or change the government and its policies,” said the paper. “They could utilize different techniques than the Islamic terrorist organizations, as many are able to blend in and use their knowledge about the homeland to increase their chances of successfully launching an attack.’’
Improvised explosives are cheaper and easier to conceal than truck bombs but can still set off powerful explosions.
Pressure-cooker bombs have been a weapon of choice in recent years in Afghanistan, said retired Army general Montgomery C. Meigs, who directed the Pentagon’s efforts to combat IEDs from 2006 to 2008. He said explosive material, such as the fertilizer ammonium nitrate, can be easily obtained. Other potential chemical ingredients could be found in a barber shop or hair salon.
“In the IED game, any household thing can contribute — cellphones, garage door openers for arming and firing,” said Meigs, who is currently president of the nonprofit Business Executives for National Security in Washington.
How the bombs were made will likely act like a signature, telling authorities several things about the person or group behind the attacks, said Alexia Ash, head of risk forecasting in North America for Exclusive Analysis Ltd.
“What we can say is that the person who made this type of weapon had some type of training or at least some type of practice,” Ash said.
Meigs agreed that identifying the content of the bombs shouldn’t prove difficult.
“It doesn’t all get blown up,” he said. “They’ll know pretty quick.’’ Meigs added that he has long feared that what was commonplace in Iraq and Afghanistan would arrive on US shores.
“Clearly something like this was in the offing,” he said. “This is just part of our brave new world.”Bryan Bender can be reached at email@example.com. Follow him on Twitter @GlobeBender