On a sweltering August day in 1992, Tammy Jo Zywicki dropped her younger brother off at Northwestern University in Illinois, and set out for Grinnell College in Iowa, excited for her senior year.
The 21-year-old never made it.
Hours after Tammy left the Chicago area and headed west toward Iowa on I-80, her white 1985 Pontiac 1000 hatchback was found abandoned on the side of the road, five miles east of LaSalle, Illinois.
Tammy — an upbeat, athletic, talented amateur photographer from Marlton, New Jersey — was nowhere to be found.
For nine days, her family, friends and the police looked for her.
Their desperate search ended on Sept. 1, when a driver spotted her body, wrapped in a blanket, on the side of a Missouri interstate.
“I was just numb,” Tammy’s mother, JoAnn Zywicki, now 78, tells PEOPLE about learning her daughter was dead.
Tammy’s oldest brother, Todd Zywicki, now 54 and a law professor at George Mason University in Virginia, still remembers the moment he heard the horrific news.
“I grabbed the phone and was standing up and I just fell on the floor,” he tells PEOPLE.
“It was just so awful and incomprehensible and, just not anything I was prepared for. I still believed that we would find her. And obviously, that hope was dashed.”
For nearly three decades, police have been hunting for answers about who killed Tammy and how her remains ended up 500 miles south from where she was last seen.
Her unsolved murder is explored in this week’s issue of PEOPLE and on the premiere of the fifth season of People Magazine Investigates. The episode, titled Highway Horrors, airs Monday, March 22 at 10 ET/9 CT on Investigation Discovery on Investigation Discovery and discovery+.
After Tammy’s body was found, the medical examiner found a strange array of puncture wounds around her heart.
She’d been stabbed with a small, sharp item which caused internal bleeding, says retired Illinois State Police special agent Martin McCarthy.
“It was a strange way to kill somebody,” he says.
Over the years, authorities pursued many leads, interviewing truckers and drivers who’d traveled through the area.
Witnesses said they’d seen Tammy on the side of the road talking to a man in a pickup truck. They also reported seeing a tractor-trailer that had pulled over near where her car had stalled.
A truck that was seen near where Tammy Zywicki’s car broke down in 1992.
While the state police declined to go into detail about the case because it’s still active, the agency said in a statement to PEOPLE, “With recent advances in DNA technology, the investigation is moving forward.”
JoAnn remains hopeful that police will find her daughter’s killer.
“When you see 40-year-old cases that are being solved, I still think there is a chance,” she says.
In the meantime, Tammy’s family has learned to live with the pain of losing her.
“She still lives in my heart and mind, even though she’s been gone from us longer than she was with us at this point,” says Todd.