On April 15, 2013, two bombs exploded at the Boston Marathon finish line — killing three people, sparking a manhunt for the culprits, and defining a specific type of terrorism that has become a pervasive threat, five years later.

“I think the Tsarnaev brothers were the face and the expression of what is the biggest current threat now — which is radicalized individuals,” news analyst Charles Sennott said during an interview with Boston Public Radio Monday. “Not trained operatives, not the ones who really fall into line with an Islamist group or some kind of organized terrorism, but those who are radicalized online, those who can be lone-wolf actors, that’s the big fear.”

After the bombing, we learned that the Tsarnaev brothers planned their attack on the marathon arbitrarily — they needed a big crowd, but the significance of Patriot’s Day, the state holiday commemorating the anniversary of the first battles of the American Revolutionary War, was not a consideration.

“They sort of stumbled upon the marathon as an idea … they were going do the fourth of July, and they just decided, 'Oh well, the marathon is this big event,'” Sennott said. “They didn’t really apparently even connect it with Patriot’s Day ... and a lot of people who interpret terrorism, like me, thought there was some connection to that date.”

“It was just a crowd and two lost kids who have more psychologically to do with Dostoyevsky than they do with Al-Qaeda,” Sennott continued. “They were just lost souls seeking violence, and they found it, terribly, five years ago.”

Sennott compared the Tsarnaev brothers’ tactics to those seen in similar “lone wolf” terrorist attacks — a different sort of strategy than one employed by an organized gang or unit.

“Increasingly, the fear is also that you have these sort of ‘sovereign citizens,’ as they are known, or lone wolves, who can come out of the very far-right fringes of white nationalism in America,” Sennott said. “I think that’s the other thing we really have got to look for, five years on — you have unbelievable numbers of high-powered weapons in the hands of a lot of people who have very warped ideas about what patriotism is in America.”

Charles Sennott is a WGBH news analyst and co-founder of The GroundTruth Project.