LITTLETON — More than three weeks after the bloodbath at Columbine High, there are still some fundamental questions that investigators cannot answer with certainty, among them: Was there a third shooter?

Some students "are very adamant about the fact that there was a third gunman," said sheriff's spokesman Steve Davis. "All along we've been trying to prove or disprove that theory."

Investigators are leaving open the possibility that another gunman joined Dylan Klebold and Eric Harris, but so far there has been no physical evidence to support the theory.

Some witnesses claimed to have seen a third gunman in a white shirt. But Davis said that one of the two teen-agers was wearing a white shirt that day and apparently took off his black trench coat during the rampage.

Ballistic tests being conducted by the Colorado Bureau of Investigation may help prove or disprove the third gunman theory.

"We match the bullets taken from the wounded and the dead and match them to weapons taken from the crime scene," said CBI Director Carl Whiteside. The tests may determine which gunman fired which shots and whether guns that are unaccounted for were used.

Whiteside said agents are testing hundreds of pieces of evidence, including bullets, bullet fragments and shell casings, and it could be weeks before the findings are in.

The findings may also help determine definitively whether the teens killed themselves or if it was a murder-suicide. Davis said the coroner's office believes both died of self-inflicted gunshot wounds.

Klebold, 17, and Harris, 18, stormed the school on April 20, hurling explosives and blasting away with their guns. When it was over, they had killed 12 other students and a teacher.

Davis said police are busy chasing down some 2,200 leads and conducting a tremendous number of interviews.

"From the outside, it looks like not much is happening. There's not big news coming out every day," he said. "But I can assure you there is a team of about 80 investigators on this task force that are very busy."

Among other things, investigators are trying to determine whether anyone else knew of the attack ahead of time or took part in some way. The large number of bombs in the school and other places — about 60 in all — suggest it may have taken more than two people to put them in place.

"From day one we've always felt like there was a very good possibility that that more people were involved," Davis said. "It may not mean that there were people actually in the school that day, but there's a good possibility that people were involved in ways such as supplying weapons or maybe building explosive devices."

Police said they have determined that Klebold's girlfriend, Robyn K. Anderson, 18, bought three of the guns used in the rampage. Investigators want to know whether she had prior knowledge of the attack. They are questioning her through her lawyer.

A fourth gun used in the attack was supplied by Mark Manes, a 22-year-old computer programmer. Manes has admitted selling the semiautomatic TEC DC-9 to Harris and Klebold.

Davis said that if it is proved that Manes or Anderson had prior knowledge of the attack and supplied the weapons anyway, they could be charged with accessory to murder.

Agents with the FBI and the Bureau of Alcohol, Tobacco and Firearms have been building a three-dimensional model of the school to reconstruct the rampage. Authorities are also working to profile the gunmen themselves.

Investigators are working with America Online to track down about 50 Web and e-mail addresses and look at Klebold and Harris' correspondence. Authorities with a search warrant went to a video store Tuesday to find out what video games or movies they rented.

"We have to reserve the possibility, however slight it may be, that we do this long, long investigation and come up with the final determination that it was Harris and Klebold completely on their own," Davis said.

"It's certainly a possibility, which would almost be a relief, to tell you the truth."

May 14, 1999 | Print this page