The officer was the lead investigator in all of the dismissed cases, Gill said.
Gill's release said "we no longer believe we have sufficient credible evidence with which to obtain a conviction. These cases were identified as part of an ongoing investigation and involve a law enforcement officer from the West Valley City Police Department."
WVCPD became aware of several issues surrounding separate criminal investigations conducted by one of its detectives and immediately began an internal investigation, the department stated in a news release.
After that, WVPCD brought their concerns about Cowley to the Salt Lake City Police Department no more than two weeks ago, and in the course of the outside investigation, Gill's office found that the scope of the problem went beyond the cases that initially concerned the West Valley City department, he said in an interview.
Blake Nakamura, Salt Lake County District Attorney chief deputy, said the office had "some concerns about the investigations and integrity of the investigations" related to the criminal cases.
Gill's office dismissed two of the cases last week, and filed motions to dismiss the remaining 17 on Wednesday.
"They were in different stages in the criminal justice system. We wanted to make sure they did not go forward, which is why we acted with the kind of urgency that we did," Gill said.
The district attorney's news release said the office is "continuing to work with the West Valley City Police Department and the West Valley City Attorney's Office to address any other collateral issues that may trigger ongoing legal and ethical responsibilities to monitor these prosecutions consistent with our constitutional obligations."
The detective is on paid administrative leave pending the investigation, said WVCPD Sgt. Jason Hauer. He has been with the department for at least a few years, Hauer said, though he did not know exactly how long.