Brittany Phillips was an 18-year-old college student in Tulsa, Oklahoma, who raised the suspicions of those who knew her when she stopped coming to classes.

He was last seen dropping a friend off at his home after 8pm on September 27, 2004.

Three days later, on September 30, officers responding to a welfare check after a concerned friend reached out to authorities found her strangled body in her second-floor apartment. She had been sexually assaulted.

In the two decades since, ex-boyfriends and strangers have been questioned, sex offenders investigated and DNA samples taken; But there is still no suspect.

“This is the first anniversary where the case is colder than it was when he was alive, because we buried him on October 4, his 19th birthday,” his mother, Maggie Zingman, 69, told PEOPLE. “I never thought he’d be killed. But then I never thought it would become an unsolved case. And then I never thought I’d be sitting here on the 20th anniversary.”

This year, as he has for the past 18 years, Zingman is traveling around the country in his bright purple and pink KIA Carnival, decorated with large paintings of Brittany and a number to call for tips.

Its purpose? To draw attention to her daughter’s case in the hope that the killer will be found.

Zingman, a trauma psychologist who works with war veterans, called his cross-country trips “The Caravan to Catch the Killer.”

“I want my daughter’s case to be solved before I die,” he says. “I feel like the cause died with me. “If I stopped, Brittany would kill me and she would do the same for me.”

A university student who loves sports

Brittany, a Tulsa Community College student, loved sports and music and was planning to transfer to Oklahoma State University and continue her chemistry studies. He hoped to pursue a career in cancer research.

Zingman last spoke to her daughter on Monday, September 27 (the same day she was last seen dropping off her friend) when the young girl had to go to urgent care for allergy issues.

“She was always going to the doctor’s office because she had allergy issues and couldn’t get in,” she says. “He was with a friend and he said to me, ‘I’m going to drop my friend off and then go home’.”

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When Brittany didn’t show up for classes, a friend called the police.

Inside the apartment, officers found Brittany dead on the floor next to her bed.

“There was some defensive equipment under the nails,” Zingman says. “He must have fought. I think he was probably waiting there or went in after she fell asleep.”

Tulsa Police Department cold case detective Jeremy Stiles says that despite interviewing hundreds of people, “no one could say anything concrete at the time this incident occurred.”

‘Maybe he’s a serial hunter’

With no suspects in sight in 2007, Zingman began to wonder if the killer had gone elsewhere to kill. “Maybe he’s a serial predator, maybe he’s traveling the highways and killing or raping people all over the United States,” he says. “I really wanted to see if there were similar murders there. Maybe a wife, a sister, a detective might hear something familiar in my story.”

“I had this idea that if I had something on the side of my car that people could see and automatically call me and bring me tips, maybe that could be a way to spread the story to the United States.” he says.

He has traveled more than 300,000 miles in 48 states over the past 18 years; distributed fliers at rest stops, police stations and traffic jams.

He crashed four vehicles, broke down in Oklahoma, Texas and Wisconsin, got lost in the middle of the night in Wyoming, drove down a Utah mountain in a torrential rainstorm, and navigated icy conditions in Washington State.

He lived without heat, water, and air conditioning to cover travel expenses, struggled from paycheck to paycheck, and went into debt before. He says he has spent nearly $120,000 seeking justice for Brittany.

“It was worth it, especially in those early years, because every time I went out I was getting 12 to 16 (newspaper) stories, and I was going out twice a year,” he says.

Caravan trips produced many clues.

“We’ve definitely had people reach out and say, ‘Dr. Zingman made me want to reach out to you and here’s what I know,’ or ‘Have you checked this?’ or whatever,” says researcher Stiles.

Stiles adds, “I have yet to come across a family of victims as committed as Dr. Zingman, and he does a great job of getting the story out and focusing as many eyes and ears on the case as possible.”

For Zingman, traveling on the open road became a way to cope with his pain.

“It’s been my way of surviving, because beyond purpose, gifts, connections, hugs, people honking their horns at me as I come down the mountain,” he says. “I’ve had so many incredible experiences. I wouldn’t have gotten through this without those experiences.”

Although he maintained hope that his daughter’s killer might one day be caught, his travels turned into connecting with families of other unsolved murder suspects.

“It’s not really about my daughter’s case anymore,” he says. “It’s really about wanting to reach out. Let me help you be a voice. Let me just say you’re not crazy. Because that’s what kills people.”

Hint complicates the timeline

A possible clue about her daughter’s murder emerged when Zingman received a phone call from her ex-husband in 2023; Her husband told her that he found a birthday card that Brittany had mailed to him in 2004 to forward to her grandfather. The card had a postage stamp dated September 29; two days after police believed Brittany was dead.

“This was a stamp from a kiosk machine,” Zingman says. “This would be easy if it was a normal stamp. Yes, it sat in the mailbox for a while, but it was printed at the kiosk. This stamp bearing that date could change the timeline.”

Stiles admits the timing is odd, saying it happened “after detectives thought the incident might have occurred.”

“I can’t say 100% what I think about it yet, but it’s unexpected and strange,” Stiles adds.

For now, Stiles is hopeful that current genetic genealogy work on the DNA found under Brittany’s fingernails will provide answers. “Obviously if this leads to one person, that’s definitely someone we’ll want to talk to from now on and find out how they got involved,” he says.

Meanwhile, Zingman is preparing for his next trip around Thanksgiving.

It plans to hit Texas, Arizona, New Mexico, Nevada and California.

“She was beautiful, she was smart, and it breaks my heart that the world has lost her,” Zingman says. “But my car was a gift. “I know millions of people know him because millions of people have seen him on the road.”

If you have any information regarding the murder of Brittany Phillips, please call Tulsa Crime Stoppers at 918-596-COPS or email [email protected].

If you or someone you know has been sexually assaulted, please contact the National Sexual Assault Hotline at 1-800-656-HOPE (4673) or go to: rainn.org.