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The 1978 cold case murder of a California high school teacher who was stabbed to death has been solved nearly five decades later, officials confirmed.

Diane Peterson was found dead on a hallway floor near her classroom at a San Jose high school on June 16, 1978, according to the Santa Clara County District Attorney’s Office. She was 26. The killing happened on the day after summer break began when teachers returned to campus to finish cleaning their classrooms, the district attorney said.

The San Jose Police Department can now confirm that Harry “Nicky” Nickerson stabbed Peterson when he was 16, after a family relative witness came forward earlier this year. The district attorney’s office said Nickerson confessed to the murder to the family member and was seen carrying a knife with the phrase “Teacher Dear” written on it.

“When we hit a brick wall and we reached a dead end with forensic evidence, it came down to old school police work and our detectives interviewing people and trying to get answers from people. And we finally got the answer we were looking for,” Deputy District Attorney Rob Baker said at a news conference Monday, June 2.

The district attorney’s office said a family member of the victim thanked investigators for continuing to search for answers, adding that “Diane was a beautiful and wonderful person who is missed dearly.”

What happened to Harry Nickerson?

Nickerson died in from a self-inflicted gunshot wound in 1993, according to the district attorney’s office. While Nickerson was never convicted in Peterson’s death, he was among the prime suspects.

A 1978 booking photo of the teen appeared very similar to a composite sketch eyewitness accounts shared, the district attorney’s office said.

“He denied being involved. He said he didn’t own a knife and the case was basically cold,” Baker said, adding that Nickerson accused officers of trying to “pin” the crime on him.

In 1983, a family of a student at the school claimed they saw Nickerson kill Peterson, but the student later denied making the statement, per the district attorney. The next year, a witness told police that they saw Nickerson carry out a drug deal that implicated the teen in the murder.

Following Peterson’s death, Nickerson was arrested and convicted of various charges including armed robbery, assault with a deadly weapon and kidnapping, the district attorney’s office said. In 1984, he was shot and critically injured during a drug robbery, but no charges were filed in that case.

Why did the witness just come forward?

The family relative, whose identity the district attorney did not share, did not come forward for decades out of fear of retaliation from Nickerson or his family, Baker confirmed with USA TODAY.

“After exhausting all other investigative leads, including extensive forensic DNA analysis, homicide detectives reached out to the relative, who revealed the secret they had been keeping for nearly 50 years,” Baker told USA TODAY. “According to the detectives, it was an emotional moment for the relative.”

The witness was not involved in Peterson’s killing in any capacity and did not help Nickerson evade arrest so criminal charges could not be filed, Baker said.

“This marks the end of a terrible and tragic mystery,” District Attorney Jeff Rosen said in a news release. “Ms. Peterson would have been a senior citizen today if she had not crossed paths with this violent teenager. I wish she was. I am pleased that we have solved this case, even though the murderer is not alive to face justice. I wish he was.”