As officials in Detroit, Michigan are beginning to test rape kits that have been sitting in police and crime lab storage facilities—some for decades—DNA evidence has already linked to multiple possible perpetrators. The Wayne County Prosecutor’s Office has identified 21 potential serial rapists from the first 153 kits that the crime lab tested and entered into CODIS, the national DNA database, according to news reports.

These kits are part of an initial wave of 200 kits that have been sent for testing under the National Institute of Justice’s grant to address Detroit’s backlog of over 11,000 untested rape kits. The DNA evidence in these 21 cases matched to the DNA profiles of suspected offenders involved in at least one other rape case, according to ClickOn Detroit. In some cases, the evidence matched to the DNA in up to five other cases, according to the article.

Testing a rape kit can identify a potential assailant, confirm a suspect’s contact with a victim, corroborate the victim’s account of the sexual assault and exonerate innocent defendants. And of course, testing rape kits can connect suspects to other crimes.

In addition to identifying the possible serial rapists, the DNA evidence in the batch of 153 kits has yielded another 38 DNA matches in the database, according to The Daily Beast. All of the cases now need to be investigated, says Wayne County Prosecutor Kym Worthy in the article.

“People think when you get a CODIS hit, we can just go out and arrest that person,” she says in the article. “But a DNA hit is never the whole case. We have to go find the witnesses, do the old-fashioned kind of investigation. They’re cold cases—they’ve just been sitting there. We have to reinvestigate all these cases.”

“I say ‘reinvestigate,’ but some were never investigated properly, frankly,” Prosecutor Worthy adds.