On the morning of July 6, 1973, Linda Anne O’Keefe woke up for summer school and put on the white dress her mother made for her, the one with the light blue flowers and the dark blue trim. She pulled her hair back in a ponytail, slipped on her matching dark blue tennis shoes with white ankle socks and headed out the door for what would be the last time.

Normally she biked to school — Lincoln Intermediate School in Corona del Mar, Calif. — but on that day her piano teacher was giving her a lift. The 11-year-old Girl Scout and avid piano player would sit through four classes that morning. But she would never come home, prompting a frantic, all-hands search with police helicopters and land crews all over the Newport Beach area.

By the next morning O’Keefe’s body would be discovered lying in a ditch. She had been strangled.

Her killer was never caught. But on Friday, the 45th anniversary of her death, the Newport Beach Police Department unveiled a new lead — albeit a small one — using DNA technology unavailable at the time that has since revived countless cold cases across the country. Before revealing it, to bring the case back to life, the police department used Twitter to recount O’Keefe’s final hours and witnesses’ final glimpses of her using details from the investigation. The tweets started about 7 a.m. Friday morning, about the time O’Keefe woke up for school, and ended just after 11 p.m., the last time someone heard her scream. They continued again as the search resumed in the morning.

The tweets were all in O’Keefe’s voice, as though she were live-tweeting the last day of her life.

“Hi. I’m Linda O’Keefe (or Linda ANN O’Keefe, if I’m in trouble with my mom),” the Newport Beach police wrote in the first one. “Forty-five years ago today, I disappeared from Newport Beach. I was murdered and my body was found in the Back Bay. My killer was never found. Today, I’m going to tell you my story.”

The afternoon she disappeared, after the final bell rang in fourth period, O’Keefe used the phone in the school office to call her mother for a ride home.

“It doesn’t go well. She is busy with a sewing project and tells me that I can walk home. It makes me upset, and I cry.”

O’Keefe moped on the curb outside the school for a while before picking herself up and trudging along down Marguerite Drive, toward home. That is when the turquoise van appeared — the same one that stopped alongside her outside Richard’s Market earlier that day.

A woman named Jannine and her mom would see the encounter unfold as they drove along Marguerite, too — “something they won’t forget for a long time.”

The van parked along the curb, near the intersection, and the front passenger door was open, with O’Keefe standing right beside it. She got in.

Suspicious, Jannine’s mom slowed down and pulled over.

“If that van drives by,” she told her daughter, according to the police tweets, “write down the license plate number.”

But the van did not drive by. It was the last time anyone saw O’Keefe alive.

At 6:42 p.m., six hours since her mother had last heard from her, O’Keefe’s parents reported her missing to the Newport Beach Police Department, convinced by then O’Keefe was not simply running off with friends to retaliate for not getting a ride. Helicopters, police Jeeps and search parties scoured the area looking for signs of O’Keefe. Only one person, a woman who did not know anything about a missing girl, was close enough to hear her, and by then it was too late.

Just before midnight, “a lady in the bluffs above Back Bay hears a female voice outside, screaming ‘Stop, you’re hurting me,’” the police tweeted in O’Keefe’s name. “She listens, but hears nothing more. She doesn’t know that I’m missing. That I’ll be dead by morning. That I’ll be found a couple hundred yards from her home.”

A man scouting the Back Bay for a good spot for a nature study was the first to find her there, just after 10 the next morning. He was looking, specifically, for frogs. When he peered through the cattails, instead he saw something “small and pale”: a hand.

“I had read about the missing girl in the paper,” he told the Daily Pilot that day. “After seeing the hand in the bushes a lot of things went through my mind. Then I knew it had to be the girl.”

According to newspaper accounts at the time, O’Keefe was fully clothed, and police did not find evidence she had been sexually assaulted. In three days, according to a 1973 Los Angeles Times article, police arrested an 18-year-old man for her murder, declining to tell reporters what evidence they had connecting him to the crime. A day later they released him from jail, saying they did not have enough.

Since then the case has remained cold — but on Saturday came the new piece of evidence: a phenotype of the person police believed killed O’Keefe. It was created by a Parabon Nanolabs, a Virginia-based DNA technology company.

Using DNA evidence, a phenotype predicts the physical appearance of a person both then and now, generating what looks like a digital avatar of a suspect. In this case, the phenotype of the suspect shows a fair-skinned man with blue eyes tinged with green and sandy blond hair. Newport Beach police say they hope it will lead to a break in the case. The man is pictured as he may have looked at age 25 and age 60.

A spokeswoman for the Newport Beach Police Department told the Los Angeles Times police decided to announce the development alongside the story of O’Keefe’s last day as a way to humanize a case that has faded from the public consciousness.

“It’s an old case from 45 years ago, and it might be hard for people to form the emotional attachment to that,” the spokeswoman, Jennifer Manzella, told the newspaper. “But we think Linda is due that.”