Nearly five decades ago, a tragic event occurred at a California high school. A young teacher was discovered stabbed to death on the school’s premises on the first day of summer vacation. Recently, cold case detectives have finally revealed the identity of her murderer.

Diane Peterson, who taught at Branham High School in San Jose, lost her life at the hands of 16-year-old Harry “Nicky” Nickerson on June 16, 1978, according to the Santa Clara County District Attorney’s Office.

The murder took place just a day after the conclusion of the 1977-1978 academic year, as teachers were tidying up their classrooms in preparation for the summer break. Peterson’s body was found by a student; she was lying on the floor near her classroom with a fatal stab wound in her chest.

Investigators cracked the cold case after meeting with Nickerson’s family member this year, according to the DA’s office. Minutes after the killing, Nickerson went to their home and confessed to stabbing the teacher, a family member admitted to police.

Nickerson had a knife with the words “Teacher Dear” written on it, investigators said.

“Just this year, investigators learned that he had confessed the killing to a family member. Nickerson committed suicide in 1993,” the DA’s office wrote.

Nickerson was not a Branham High School student, Deputy District Attorney Rob Baker told Nexstar’s KRON. While investigators have never established a firm motive, “detectives at the time theorized that Diane inadvertently interrupted a drug deal on campus the day after school let out for the summer,” Baker said.

District Attorney Jeff Rosen said, “This marks the end of a terrible and tragic mystery. 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.”

Nickerson was arrested four days after the killing for an unrelated incident, Baker said. “He spontaneously accused the police of trying to ‘pin’ the murder on him, he denied killing Diane, and denied owning a knife,” Baker said.

Police took a mugshot of the 16-year-old boy in June 1978, and said it bore a strong similarity to a composite sketch based on eyewitness accounts of the high school attack, prosecutors said.

In 1983, a Branham High School student’s family alerted police that their son claimed to have seen Peterson’s violent death and identified Nickerson as the person responsible. The student, however, later denied making that statement. In 1984, another witness told police that Nickerson confessed to killing Peterson, but police were unable to corroborate the claim.

After high school, Nickerson continued committing more crimes, including kidnapping, assault with a deadly weapon, and armed robbery, prosecutors said.

“He was shot and critically injured in 1984 while attempting to commit a drug robbery. Given the circumstances, no charges were filed. He died in 1993 from a self-inflicted gunshot wound,” teh DA’s office wrote.

Between 2023 and 2024, the DA’s Crime Lab conducted extensive DNA work on the case in an attempt to identify Peterson’s killer. Unlike other recently-solved cold cases in the area, forensic investigators were unable to make a DNA match. Instead, the break in the case came from Nickerson’s family member coming forward to police.

“Nearly five decades have passed since a young teacher’s life was tragically taken,” San Jose Police Chief Paul Joseph said. “While the suspect will never stand trial or face the consequences for his actions, we hope this resolution brings a measure of peace to the victim’s loved ones and to a community that has carried this loss for far too long. No matter how much time passes, we will continue to seek the truth. Every victim matters, and every life deserves justice.”

The DA’s Cold Case Unit was established in 2011 and has solved over 30 cold case murders from as early as 1969. The Peterson case is the fourth cold case homicide solved by the office in 2025.

A family member of the teacher said, “Diane was a beautiful and wonderful person who is missed dearly.”