Why would Robyn Anderson buy three guns?

She was exceptionally bright, taking tough classes such as calculus and Advanced Placement English at Columbine High School.

She had a 4.0 grade-point average and was in the running to be Columbine’s valedictorian, said her best friend, Tiffany Burk, 18, a senior.

And she was identified this week as the gun-buying prom date of Dylan Klebold, 17. He and his pal Eric Harris, 18, used the guns to kill 13 people and themselves in the April 20 Columbine High School killings.

On Wednesday, authorities narrowed the connection between Anderson, 18, and the guns.

“We have confirmed at this point that she did buy three weapons, those being the long guns — the two shotguns and the rifle,” Jefferson County Sheriff’s Department spokesman Steve Davis said.

Anderson was in the school parking lot when the shooting started, possibly headed to lunch, Burk said. Anderson told her friend she spent the aftermath of the shootings huddled in her car for three hours.

“She was actually in the parking lot underneath her car seat because she heard firing,” Burk said.

Anderson lives with her mother. Her parents are divorced and she has two older sisters, one a 1997 Columbine graduate who is now in college.

Burk said Anderson and Klebold were pals, confidants. Their relationship was close, but not romantic, she said.

“They were incredibly kind to one another. Dylan had tons and tons of respect for Robyn,” Burk said. “She was someone who he confided in.”

Anderson and Klebold joined several couples in a limousine for Columbine’s April 17 prom.

Anderson told her friend they had a “wonderful time … and Dylan was being a little more chivalrous that night,” Burk said.

Burk, who’s been close friends with Anderson since the two met in freshman geometry, said she cannot fathom that Anderson knew of the deadly plans by Klebold and Harris when she purchased the guns.

The shootings left her unbelieving, Burk said. “She was very shocked, completely stunned.”

She recalled Anderson and Klebold joking around about how “the jocks think they owned the school.”

Other students say Anderson is shy, but pleasant. Normal. Now that authorities have named her as the source of most of the firepower, she has vanished.

So far, police say, she’s a witness, not a suspect.

Supplying guns used in a killing is a violation of federal law. If the buyer knew the guns were going to be used in a crime, it’s a felony.

Agents from the Bureau of Alcohol, Tobacco and Firearms said that federal and state prosecutors would work to determine how to handle possible charges against Anderson, but nothing is final.

Troubling questions remain about how much Anderson knew.

In Columbine’s senior class photo, she and Klebold have their arms around each other’s shoulders, laughing wildly.

Stephanie Hommel, a junior, was in German class with Anderson and had shared hotel rooms with her during a class trip to Munich in March.

“She didn’t strike me as a person who would go out and buy guns,” Hommel said.

Anderson and Hommel joked around a lot on the trip, talked about everyday stuff like boys.

John Savage, a senior who narrowly escaped being shot that day, also knew Anderson. “She was always pretty nice to me,” Savage said.

“I don’t think she would have bought them (the guns) if she would have known what they were going to be used for,” Savage said.

Why?

“I don’t think she hated Columbine.”

Anderson’s mother, Kay Anderson, teaches elementary school for Littleton Public Schools, spokeswoman Karla Langton said. Langton refused to identify the school, on advice of an attorney.

Neighbors of the Andersons say she and her mother had lived in their home a year or two.

Residents mostly knew of the family because parents of young children got angry about teens, especially boys, speeding in and out of Anderson’s driveway.

Other than that, there was never anything suspicious about her, they said.

“She looks as normal as any teen-ager, long blond hair, whatever,” said a neighbor who asked not to be named.