The suspect is dead, the fertility clinic where he reportedly replaced a customer's sperm with his own no longer operates, and the clinic left no records that might show the extent of the scandal, the University of Utah said Tuesday.
A university hotline has received 17 calls in recent days about possible tampering with semen samples at the clinic once operated by faculty members.
Officials say they may never get to the bottom of a complaint that a convicted felon working at the clinic two decades ago switched his sperm, leaving a now 21-year-old woman asking, "Who am I?"
The mix-up happened in 1991, the clinic shut down the following year and the suspect died in 1999.
"Unfortunately, the reality of this very disturbing situation is that there is very little information with which to make any definitive conclusions," Kathy Wilets, a spokeswoman for the University of Utah's health sciences division, told The Associated Press in a statement.
She added, "We believe it is impossible to determine exactly what happened."
State and federal prosecutors said they were unaware of the allegation and weren't certain it warranted an investigation.
The mother of the 21-year-old woman, Pamela Branum, has said she and her husband discovered a genetic mismatch in their daughter and were able to trace her lineage with help from relatives of the now-deceased fertility clinic worker, Thomas Ray Lippert.
Branum said she believes the university hasn't conducted a serious investigation and is trying to conceal the possibility of a wider scandal.
Wilets said the university is sharing as much information as it can offer the family.
The University of Utah had no ownership interest in the clinic, Reproductive Medical Technologies, but used some of its services. Three of the clinic's owners were faculty or staff members, and surviving partners have declined comment.
The U.S. Food & Drug Administration regulates fertility clinics, but said it has been only a decade since regulations governing the handling and storage of reproductive tissues started taking effect.
An FDA spokeswoman couldn't immediately explain how semen banks are supposed to handle samples to avoid mix-ups.
"I thought I was this person (of) my mom and my dad," the family's daughter told CBS affiliate KUTV. "Now, my dad is not my biological father. Who am I?"