PITTSBURGH -- Target 11 investigates a missing person's case that began 21 years ago. In 1981, a 16-year-old Zelienople girl vanished. To this day, police believe foul play was involved.

Michelle Reidenbach was 16 when she disappeared. Pictured right, is what she looked like then.

She would be 37 today and below is a computer generated picture of what Michelle would look like today.

Jeff Reidenbach said, "That's the hardest thing, not knowing one way or another. I think it's stressful. It's something you think about every day."

Jeff is talking about Sept. 22, 1981, the day his 16-year-old sister, Michelle, disappeared.

Jeff said, "Really there wasn't a whole lot you could do. Mostly police did it. If they asked you to do something you did it, but not a whole lot you could do. When they checked with her friends, you don't know anywhere else to look."

Target 11's Rick Earle asked, "Do you think she ran away?" State Police Cpl. Ray Melder said, "No. I don't believe so, nor does any investigator involved with this case."

Melder has been working the case for the past decade. He keeps a picture of Michelle, along with a timetable of events from the day she disappeared, on the wall of his office at the Butler barracks.

Melder said Reidenbach left Seneca Valley High School around 3 p.m. and drove with several friends to Zelienople.

She spent time with a girlfriend at a gas station and around 4 p.m. walked across the street to the appliance store where she worked.

About an hour later, she left the appliance store and walked about a block away to a Rite Aid drug store to buy medicine for a cold.

Melder said, "The clerk at the Rite Aid does recall seeing Michelle come into the store purchase the cold medicine. That was the last time we are certain of that she was seen."

Fred Landay said, "She was a good girl. She was a nice girl."

Landay was the owner of the appliance store where Michelle worked.

Landay said, "I didn't see her that day. She was working upstairs doing envelopes for a private sale and she just never came back. She left her purse and just never came back."

Over the years, police questioned several people, including Reidenbach's boyfriend. Police ruled him out after he passed a lie detector test.

Recently, the Center for Missing and Exploited Children released an age-enhanced photograph (pictured, right) of what Michelle would look like today. Jeff said, "Sometimes you wonder if you want to know if it's bad, you know but after this long you don't have a lot of hope. Until you find something out you hope that that will be what happens."

Police told Target 11 over the years they continue to get new information and leads and every time they follow them up.

If you have any information about this case you're asked to call police at (724) 284-8100.