An independent investigation concluded Wednesday that a student killed in the Columbine High massacre was slain by one of the teen gunmen and not by a police officer, as his parents had alleged.

The report found that Denver Sgt. Daniel O'Shea was not present at the time that 15-year-old Daniel Rohrbough was shot. The report also concluded that a bullet from the Rohrbough's body was a "textbook match" to Eric Harris' gun.

"The murder of Daniel Rohrbough at Columbine High School on April 20, 1999, was undeniably caused by gunman Eric Harris beyond any reasonable doubt," the report said.

Rohrbough was among 12 students and a teacher killed by Harris and Dylan Klebold during the rampage in suburban Littleton. Harris and Klebold then killed themselves in the school library.

Rohrbough's family had alleged in a lawsuit that O'Shea had mistakenly killed their son -- something that investigators had denied all along.

Brian Rohrbough, Daniel's father, said the report gave the family some satisfaction. "I believe for the first time in three years we have the very first plausible explanation for what happened to my son," he said.

Barry Arrington, an attorney for the Rohrbough family, said investigators had access to information that has been denied the families. He also said Rohrbough's relatives will review the full report before deciding on their next step.

The investigation was conducted by the El Paso County sheriff's office and detectives from Colorado Springs, at the request of the Jefferson County sheriff's department, the lead agency in charge of responding to the attack. El Paso County's deputies had no role in the initial response to the attack.

A private lab in Maryland conducted tests that matched a 9mm bullet from Rohrbough's abdomen to Harris' weapon, the report said. Rohrbough was shot before 11:30 a.m., based on witness accounts, while O'Shea arrived on the scene just before noon, according to the report.