(CBS) – A former state investigator believes he knows who killed Tammy Zywicki, more than two decades after the college student was abducted and murdered.

“We can identify, I believe, the killer,” Martin McCarthy tells CBS 2’s Mike Parker.

The retired Illinois state police master sergeant was a member of the task force investigating the 1992 killing. Although he’s been off the job for 13 years, he hasn’t given up the search for her killer.

“This is not my first rodeo. I’ve done homicides for many, many years. The most obvious guy is the guy who did it,” he says.

McCarthy believes it was Lonnie Bierbrodt, now dead, a felon on parole who kidnapped the 22-year-old college student from alongside her disabled car on Interstate 80 near LaSalle, raped and killed her in his truck and then dumped her body near Interstate 44 near Springfield, Mo. — not far from Bierbrodt’s home.

McCarthy says Bierbrodt has been placed in  LaSalle County on the day Zywicki disappeared, and that a witness has identified Bierbrodt as the man she saw standing alongside the victim along the highway. He also says that Bierbrodt had his truck steam-cleaned and sold it not long after the murder.

He won’t let the case go.

“I think that murder is forever. We never, ever stop,” McCarthy says.

There’s something else. One witness says Bierbrodt gave away, as a gift, a key piece of evidence: an unusual music box wristwatch, the same kind of watch that was missing from Zywicki’s suitcase. Tammy Zywicki was on her way to Iowa where she attended college.

Bierbrodt died in 2002. McCarthy says he hopes authorities finally see the case his way, and close it after all these years.

The Zywicki family, he says, needs some closure.