Police on Kauai investigated allegations officers were involved in cheating on a proficiency test and covering it up, a report said.

A May 2 letter from county prosecutors to Honolulu attorney Michael Green briefly described the investigation, saying one officer allegedly gave another officer a copy of a test before the second officer took it. Four other officers allegedly “knew and covered it up,” the letter said, The Garden Island newspaper reported Thursday.

Green represents a Kauai man facing charges related to a hit-and-run incident last year. Green filed a memo in court saying the investigation may affect whether the state will proceed to trial. The officers are potential witnesses in his client’s case and their credibility is paramount in resolving it, he said in the memo.

A county spokeswoman confirmed the Kauai Police Department conducted an internal investigation but couldn’t provide further details because it involves an ongoing personnel matter.

In an interview last week, Green said police investigations into other cases could be undermined if Kauai police were caught cheating and their ethics were called into question.

The existence of the investigation came to light in December, when the county prosecuting attorney sent a letter to Emmanuel Guerrero, a lawyer defending a man facing negligent manslaughter charges. The prosecutor explained the police had launched an investigation into officers’ handling of his defendant’s case. The letter didn’t provide details on the allegations, just that it “concerns matters that may affect the credibility” of four officers.

In the months since, prosecutors sent similar letters to two other defense attorneys naming two other officers.

Guerrero filed a motion in court asking Fifth Circuit Court Judge Randal Valenciano to force prosecutors to turn over documents related to the internal investigation. Prosecutors said they had asked police for the documents but the police refused to comply.

Court records give few clues about how the investigation was conducted or what it revealed.

According to the minutes of a February court hearing, Deputy Prosecutor Peter Morimoto said a report of the investigation had been completed and was pending a determination to be made by the police chief at an administrative hearing to be held two weeks later.

Police may never have to disclose whether their officers were involved in a cheating scandal unless investigation documents are presented as evidence at trial.