Brittany Phillips was an 18-year-old student in Tulsa, Okla., when she stopped showing up to class, raising the suspicions of those who knew her.

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

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

In the two decades since, ex-lovers and strangers have been questioned, sex offenders investigated and DNA swabs taken – but still no suspects.

“This is the first anniversary where the case was colder than when she was alive because we buried her on her 19th birthday, October 4,” said her mother Maggie Zingman, 69 for years, for PEOPLE. “I never thought she would be killed. But then I never thought it would be a cold case. And then I never thought I’d be sitting here on my 20th birthday.”

This year, as she has for the past 18 years, Zingman is traveling the country in her purple and bright pink KIA Carnival, which is emblazoned with large pictures of Brittany and a number to call for advice.

Her goal? To draw attention to her daughter’s case in hopes of finding the killer.

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

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

A student who loved sports

Brittany, a student at Tulsa Community College, loved sports and music and planned to transfer to Oklahoma State University and continue her chemistry studies. She hoped for a career in cancer research.

The last time Zingman spoke to her daughter was Monday, September 27 – the same day she was last seen dropping off her friend – when the teenager had to go to urgent care due to allergy issues .

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

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

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

“There was some defensive stuff under the nails,” Zingman says. “You must have fought. My belief is that he was probably waiting there, or came in after she was asleep.

Despite interviewing hundreds of people, “no one has really been able to give anything concrete as to when this happened,” says Detective Jeremy Stiles of the Tulsa Police Department.

“Maybe he’s a serial predator”

In 2007, with no suspects in sight, Zingman began to wonder if the killer had moved on and was killing elsewhere. “Maybe he’s a serial predator, maybe he’s traveling along highways and he’s killed or raped people in the US,” she says. “I really wanted to see if there were similar crimes out there. That maybe a husband, a sister, a detective would hear something familiar in my story.”

“I had this idea if I had something on the side of the car, something that people could see and automatically try to look me up and bring advice, maybe that would be a way to get the story across in the US.” she says.

Over the past 18 years, she has traveled more than 300,000 miles in 48 states, handing out flyers at rest stops, police stations and traffic jams.

She went through four vehicles, had cars break down in Oklahoma, Texas, and Wisconsin, got lost in the middle of the night in Wyoming, whitewashed a mountain in Utah in a severe storm, and navigated icy conditions in Washington state.

To pay for the trips, she lived without heat, water and air conditioning, struggled paycheck to paycheck and previously went into debt. She says she has spent about $120,000 in her quest for justice for Brittany.

“It was worth it, especially in those early years, because I’d get 12 to 16 stories (in newspapers) every time I went out, and I went out twice a year,” she says.

Her RV trips have generated several tips.

Stiles, the investigator, says, “I’ve definitely had people say, ‘Dr. Zingman made me want to contact you, and here’s what I know,’ or ‘Did you check that,’ or whatever.”

Stiles adds, “I have yet to meet another victim’s family that is as determined as Dr. Zingman, and she does an amazing job of bringing the story and getting as many eyes and ears on it as possible.”

For Zingman, traveling the open road was a way to deal with her pain.

“That’s how I survive, because beyond the purpose, the gifts, the connection, the hugs, the people honking at me coming down a mountain,” she says. “I had such incredible experiences. I could not have survived without these experiences.”

Although she remains hopeful that one day her daughter’s killer will be caught, her travels have become linked to other homicide families.

“It’s not about my daughter’s case anymore,” she says. “It’s really about wanting to get there. Let me help you be a voice. Let me say that you are not crazy. Because that’s what kills people.”

The advice complicates the timeline

A possible lead in her daughter’s murder emerged in 2023, when Zingman received a call from her ex-husband, who said he had found a birthday card Brittany mailed to him in 2004 to forward her grandfather. The card was postmarked September 29 – two days after police believed Brittany died.

“It was a stamp bought from a kiosk machine,” Zingman says. “If it was a regular stamp, it would be easy. Yes, it sat in the mailbox for a while, but it was printed by the newsstand. This stamp with that date could change the timeline.”

Stiles agrees that the timing is odd, saying it’s “after what the detectives thought the incident would have happened.”

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

Right now, Stiles is hoping that the current genetic genealogy work on the DNA found under Brittany’s fingernails will provide answers. “Obviously, if it leads to an individual, that’s definitely someone we’ll want to talk to further and figure out how they were involved,” he says.

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

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

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

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

If your 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.