Updated 08/22/12 - 8:07 p.m.
CHICAGO (CBS) -- The mother of a murdered college student says 20 years later, she's resigned to the loss of her daughter but still wants to know who killed her.
Joanne Zywicki said her daughter Tammy was just 21 when she was kidnapped from an Illinois highway. Twenty years ago on Thursday, her car broke down as she was driving back to college in Iowa.
"She has so much to give and so much to offer," said Joanne Zywicki.
LISTEN: WBBM Newsradio's Regine Schlesinger Reports
Witnesses had seen a truck that had pulled over, apparently to help her. She wasn't seen alive again.
A few days later, her body was found along a road in Missouri.
Joanne Zywicki, who now lives in Florida, said it is "tough" on her on this 20th anniversary.
"It just brings back a lot of memories, a lot of everything that happened. Most of all it's hard because we don't have any closure," she said.
"We would just like to know," she added. "We'd like to have some answers. We know there is somebody out there who knows something."
Over the years, police focused on two truckers as possible suspects but were never able to link either definitively to Zywicki's murder.
Chicago FBI spokesman Ross Rice told CBS 2's Mike Parker it's hard to believe that 20 years have gone by since Zywicki's disappearance and murder.
"And it's very unfortunate," he said, "that we haven't been able to solve the case during that period of time."
Ross headed up the initial investigation, and said the case is personal for him and other agents.
"Every August, about this time of year, I think back to 1992 as kids are going off to school. As my kids became college kids, and they left for school ... I would invariably think of Tammy Zywicki," he said. "It left a lasting impression on me as it did anyone who was associated with the case."
for more features.