But if the pair's fondness for Hitler and black trench coats seemed bizarre, other students could relate to the social pecking order that had put Klebold, 17, and Harris, 18, squarely at the bottom.
High school can be cruel if you're not part of the "in" crowd, and Klebold and Harris were reminded daily that they were "out."
"Our school is very status structured. People would yell comments to them, like weirdo or outcast," said Johnna Nelson, 17, a junior. "They'd blow it off, but over and over, it must have had an effect."
On Tuesday, Klebold and Harris walked into their school carrying guns and explosives and killed 13 people. Witnesses say they singled out those who infuriated them the most: athletes who occupied the top of Columbine's social ladder.
"All jocks stand up! We're going to kill every one of you," one of the gunmen yelled in the library, recalled Aaron Cohn. He believes he was spared because a girl leaped onto his back while he lay on the floor, covering the baseball slogan on his shirt. Minutes later, the gunmen turned their guns on themselves.
Both teen-agers' families issued statements Wednesday expressing sympathy for the victims, and Klebold's family wondered what went wrong: "Like the rest of the country, we are struggling to understand why this happened," the family said.
Harris and Klebold had a juvenile record, but not for anything violent -- they were caught breaking into a car last year.
But the assault did not come out of the blue. The two were part of a dozen or so misfits who called themselves the Trenchcoat Mafia, a group well known around school.
Members posed for a yearbook photo last year. They had their own special spot in the cafeteria, near the stairs. They wore black trench coats -- no matter the season -- and berets with German crosses. They openly admired Hitler. They spoke constantly of war and guns, and Harris had made a video at school in which he bragged about his new guns.
Harris was a leader in the group, other students say. He wore German insignia, spoke German in the hallways to Klebold, and loved violent computer games. He told classmate Andrew Beard that he intended to join the military after graduation.
"He was into combat more than anyone else I've ever seen," Beard said.
From 1993 to 1996, Harris lived near the Canadian border at the Air Force base in Plattsburgh, New York, where his father, Wayne Harris, was a pilot, according to the Plattsburgh Press-Republican. In Littleton, the Harris family moved into a tidy, two-story home on a quiet suburban= cul-de-sac.
Friends of Klebold said he went astray after meeting Harris.
"Dylan was real quiet, real smart," said Nick Baumgart, 17, who met Klebold in grade school. "When Eric and Dylan got together, Eric changed Dylan's demeanor."
At school, they were respectful of teachers, reserving their invectives for blacks, Jews, Hispanics, and especially athletes. Klebold was a Boston Red Sox fan, but his enjoyment of sports did not extend to the "jocks" in school who taunted him.
"Dylan said he hated the jocks, and how they could walk over people and thought they were tough," Beard said.
A couple of months ago, members of the Trenchcoat Mafia made a date to fight the jocks on a Friday night at a baseball field, said football player Matt Good.
The jocks showed up, but the Trenchcoats were two hours late, and they went to the wrong spot, Good said. They also showed up carrying swords and brass knuckles -- not the jocks' idea of a clean fight. The fight was never rescheduled.
John House, 17, a senior, said he refused to associate with Klebold after he joined the Trenchcoat Mafia.
"I went bowling with him, and when he would do something good, he would shout, 'Heil Hitler!' and throw up his hand," House said. "It just made everyone mad."