The parents reportedly suspected him. Investigators said the murder appeared to be "personal" in nature...
______________
"Wounds on the teen’s hands showed she fought her killer, but the medical examiner’s office said Michelle was found fully clothed and had not been molested.
Detectives found no weapon or fingerprints to identify her killer, but said Michelle had not been robbed.
Based on the number of stab wounds – particularly to the young woman’s face – police considered the homicide personal in nature."
Albert and Janet Martinko died believing they knew who killed their daughter, but there was no evidence tying that man to the crime.
https://www.thedarksideofamerica.com/unsolved-michelle-martinko.html
The story says the parents suspected an ex-boyfriend, but that ex-boyfriend was not this guy.
____________
"Before the murder, Janet Martinko had been a lively, outgoing woman. Afterward, she didn’t want to be seen in public. She was reluctant even to go to the grocery store. Both Martinkos plunged themselves into painful seclusion and suffered from health problems until their deaths — Al in 1995, Janet in 1998. They went to their graves believing an ex-boyfriend of Michelle’s had killed her, Stonebraker said.
It turned out they were wrong.
In 2006, Doug Larison was the cold case detective for the Cedar Rapids Police Department. He got a tip about a suspicious person connected to the 27-year-old case. Though the tip didn’t produce any solid leads, it gave Larison the chance to take another look at the case file.
He found something no one else had found: The killer’s blood. Though he won’t say where he found it, or how he knows it belongs to the killer, Larison is certain the blood came from a cut on the killer’s hand, and that the killer was a man.
Police had the state crime lab extract DNA from the blood and file it in the national computer database of DNA profiles. Of 80 suspects, 60 have been tested and none has been a DNA match. The ex-boyfriend was exonerated."
https://www.thegazette.com/2009/12/20/30-years-ago-murder-of-teen-haunts-those-who-knew-her