The explosive devices that tore through the final mile of the Boston Marathon on Monday were not designed to inflict maximum physical damage, experts say, but rather to convince Americans they are never safe.
Law enforcement officials believed it was only a matter of time before the improvised explosive devices that have defined the conflict in Afghanistan, Iraq and even Northern Ireland would be used against Americans on their home soil. Military bomb technicians from the U.S. Navy and U.S. Army National Guard have mobilized to Boston to contribute to the investigation.
"Most people in law enforcement believed we would see these IEDs begin emerging in the U.S.," says Craig Dotlo, a retired FBI agent who helped investigate the Sept. 11 attacks in New York. "Sure enough we saw it yesterday."
"This is really an attack on the country. It's not just a law enforcement problem," he adds. "This seems to be a pattern of activity – if it turns out to be international terrorists – who really want to change our way of life and our culture."
President Barack Obama confirmed at a press conference early Tuesday that two bombs were purposefully detonated as an act of terrorism, but reinforced that investigators still do not have any sense of the motive of who conducted the attacks.
These bombs, unlike the 1993 and 2001 attacks on the World Trade Center, were not designed to inflict structural damage on the surrounding buildings. Dotlo believes Monday's explosions were designed to kill as many people as possible and to send a message: "We can get you wherever you are. You can't go any place that is safe."
Initial reports that investigators had found an unexploded bomb proved false. This would have yielded a significant amount of information, Dotlo says. FBI agents would have an advantage of finding evidence in one place instead of scattered throughout the subsequent blast area. They could also try to re-engineer the device and trace its components to where they were acquired.
Eyewitness testimony and other evidence from surveillance should yield some sort of "solution to the problem" in the not too distant future, he says.
One of the key components to the investigation is determining how and when the bomber deposited the device. Both experts agree that Boston police and other security officials could have done little to prevent this from happening.
"You can't fault security," says Paul Fennewald, a retired FBI bomb technician. "If we're going to have any type of an open society whatsoever, you're going to have things like this."
Some bombs can be made out of ordinary household chemicals and detonated with a match head, making them virtually untraceable before they explode, he says. "There's only so much you can do."
Dotlo points to the physical sprawl of a spectacle that attracted so many participants and spectators.
"The marathon is a 26-mile event. This bomb could have gone off any place along the route," he says. "It's hard enough to secure an arena or a sports venue, but when you're talking about a route that goes 26 miles, you can imagine how difficult that would be."
Both experts reinforce the Department of Homeland Security advice: "If you see something, say something." Reporting suspicious activity can tip law enforcement officials that something might not be right.