Shared paranoid disorder might also explain why the two brothers did not initially plan for a quick getaway after the bombing. “They might have fantasized that God would take care of them,” Bursztajn said, and enable them to escape being identified as the bombers. Or they might simply have been careless in their planning.

The psychiatric condition, however, is uncommon, and a far more plausible scenario is that the brothers were emboldened by each other.

“They may believe that murder is wrong, but their sense of allegiance and loyalty to each other or the group may supersede that sense of right and wrong,” said James Alan Fox, a criminology professor at Northeastern University.

“Awful crimes may be committed just for the sake of a perverted kind of bonding,” Fox said. Criminals often compartmentalize, dividing the world into those they care about and everyone else.

And just as some men who commit group rape would never rape someone on their own, some people only kill in pairs. One brother may have been looking for his younger brother’s admiration, while the other was looking for his older brother’s approval.

“I think they brought out the worst in each other,” Fox said. “I’m not sure either would have committed murder on his own.”