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Those who sent their children back to school did so with reluctance, and many had to be persuaded by their youngsters that it was time to return to the routines of life.

“It was hard, but she doesn’t want to hear about it anymore. She wants to get back to normal,” said John Gentile of nearby Wolcott, explaining why he sent his 12-year-old daughter, Julia, back to school.

As complicated as the gun-control issue is, the people of Newtown and psychologists are grappling with the more difficult question of why Lanza and others do what they do and what can be done to prevent it.

Those who knew the shooter, who took his own life as first responders closed in on Sandy Hook, say he was odd and shy, but that they never imagined what he was capable of doing.

“To know there was a hidden evil walking around school, just another face in the hall, is heartbreaking,” said 19-year-old Brendan Delohery, who graduated a year after Lanza.

Neil Berthier, who also knew Lanza during his high school years, described him as a loner who rarely associated with classmates. After learning who the killer was, Mr. Berthier said, he wasn’t surprised but until then never sensed Lanza would resort to such heartless violence.

“Once you realized who [the killer] was, it all made sense,” Mr. Berthier said. “He really didn’t have any friends. … But you never think they’re going to do anything like that.”

Mental health professionals confront the same problems. The mind of someone like Lanza, they say, largely remains a mystery.

“Human behavior is very complex. … What we can say with confidence is that this individual was suicidal,” said Richard Shadick, director of a counseling center and a psychology professor at New York’s Pace University. “Individuals who are suicidal, there is only a very small subset of those who are violent. We’re looking at a very rare phenomena here that we don’t know a lot about.”

That rare phenomenon was on full display Friday at Sandy Hook and, like Mr. Shadick and other specialists, the people here can’t explain it.

Jim McCloskey, a Vietnam veteran who came to Newtown’s memorial in the town square Tuesday, said he can understand why someone would struggle with thoughts of suicide. He can almost understand, as twisted as it is, why someone may consider killing a family member — in Lanza’s case, his mother.

But Mr. McCloskey is asking the harder, more unsettling question that Newtown and the nation as a whole want answered.

“Why did you have to kill those kids? We’ll never know,” he said.