For more than three days — from the time of the explosions at 2:50 p.m. on Monday, April 15, until the F.B.I. released their photographs to the world at 5 p.m. on Thursday, April 18 — the very ordinariness of their activities let the brothers hide in plain sight…

Slipping back into a routine after committing a crime, even an atrocity, is fairly typical behavior, said Dr. Stuart W. Twemlow, a retired professor of psychology at Baylor College of Medicine in Houston. He works on threat assessment with the F.B.I. and helped on the Columbine shootings, among other cases.

A return to business as usual helps a criminal “blot out the horror with which he was associated,” Dr. Twemlow said.

“That is a normal, dissociative response,” he said, adding that the younger brother, whose movements were more public, had most likely “denied and compartmentalized what he had just done.”…

“I stopped by his room a couple times,” Mr. Juozaitis said. “He was just playing FIFA on Xbox. It wasn’t, like, weird. He was just doing what I do.”