The lone-wolf terrorist poses special nightmares for security and intelligence agencies.
Nothing known so far about the twin bombs in Boston that killed three and maimed dozens required more resources than what's easily available to a single crazed individual.
In terms of cost, logistics, sophistication, placement, and timing, a single terrorist – a lone wolf – could have built, tested, and planted the pair of kitchen pressure-cookers crammed with gunpowder and laced with nails to killing and maim innocent spectators lining Boylston Street.
The two bombs, at least one of which was left in an unremarkable, black, nylon gym bag, could easily have been carried by a single person and left about 200 metres apart, after the final explosives sweep of the race route roughly an hour before the lead runners passed.
Lone-wolf terrorists, especially if they are careful and willing to learn from the mistakes of others, are nearly impossible to detect. There is no network for intelligence agents to intercept, to monitor.
"By far, lone wolves are the biggest concern to security agencies," says Christian Leuprecht, a terrorism expert who teaches at both Queen's University and Royal Military College of Canada in Kingston.
While international Islamic jihadist groups – including al-Qaeda – have urged ideological fellow-travelers to self-train to carry out indiscriminate terrorist attacks, lone wolfs typically are driven individuals with their own agendas.
But lone-wolf terrorism isn't rare. A Dutch terrorism database classified roughly 40 per cent of all terrorist attacks as "lone-wolf" strikes, Dr. Leuprecht said.
As for pressure-cooker bombs, they are common. "We are defusing pressure-cooker bombs almost daily," Shafqat Malik, chief of the bomb disposal squad for Pakistan's Khyber-Pakhtunkhwa province, which includes violence-wracked Peshawar and the Swat Valley, told Reuters.
Plans are widely available online. "How to Make a Bomb in Your Mom's Kitchen," featured pressure cooker bombs and was published in the al-Qaeda backed English-language Inspire magazine 2010 edited by Anwar al-Awlaki, the radical Muslim cleric and U.S. citizen killed by a Hellfire missile in a 2011 drone attack.
The Boston bombs apparently used ordinary gunpowder or homemade explosive – nowhere near as powerful as plastic explosives but easily available and inexpensive if extracted from shotgun shells or mixed in a basement. Both bombs, including timers and explosive could have been built for a few hundred dollars."
A timer using a digital watch and a battery-powered light bulb filament can serve as a detonator.