While the younger brother has been described as good student and warm-hearted, his behavior may have been influenced by his older brother, said James Alan Fox, a professor of criminology, law and public policy at Northeastern University, one of the Boston colleges that remained closed on Friday.

“It’s not just an anti-American or Islamic jihadist ideology, it’s the relationship between the two perpetrators,” said Fox, who was interviewed by telephone while he was locked inside his Boston home. “They brought out the very worst in each other. Individually, they may have seen like nice guys. But together they create a different entity.”