Philadelphia prosecutors on Monday dropped charges connected to one of the four men who accused a former city homicide detective of sexual assault, saying they’d been unable to find the witness in recent weeks and didn’t know if he would be available to testify at trial.

Assistant District Attorney Brian Collins said prosecutors had unsuccessfully sought to contact the man through his parole officer, and at one point even set up a drug-related sting to try and find him.

“We’ve not been able to locate him,” Collins told Common Pleas Court Judge Giovanni Campbell. The judge agreed to let prosecutors drop more than half the charges against former detective Philip Nordo, including counts of rape, institutional sexual assault, and official oppression.

The decision came as prosecutors and Nordo’s defense lawyers began selecting jurors for his trial. The DA’s Office has accused Nordo of sexually assaulting men he met during more than a decade in the Police Department’s homicide unit, and they’re still expected to present three witnesses at trial who have accused Nordo of forcing himself on them during investigations.

Nordo has consistently denied the allegations, and his lawyers have said he did nothing wrong. Monday morning, they unsuccessfully sought to persuade Campbell that the charges involving the three remaining witnesses should be addressed at separate trials, saying the men’s disparate accusations were unrelated.

“There’s no linkage there,” said attorney Richard J. Fuschino Jr.

Campbell denied the request.

The dropped charges relate to a man who had accused Nordo of numerous instances of sexual assault, prosecutors said: First beginning in 2003, when the man, then 18, had agreed to work as an informant, and again beginning in 2012, afterhe said he could provide information about a homicide.

The man — identified in court documents only as Complainant #3 — accused Nordo of forcibly performing oral sex on him in cars and jail visiting rooms, the documents said, including while the man was handcuffed.

Nordo’s lawyers asked Campbell to bar prosecutors from re-filing charges if they ever find the witness, but the judge denied that request.

The remaining witnesses, according to court documents, include a man who said Nordo raped him in a Chinatown hotel room, and two men who say Nordo harassed them and later attempted to sexually assault them — one in a car, the other in an elevator at Police Headquarters.

Jury selection was expected to continue Tuesday. Campbell said Monday that testimony would likely not begin until early next week.

The trial is expected to last several weeks.