The Philadelphia Daily News first reported the story Monday.
Reached at his Bust–leton home Monday, Rossiter declined to comment.
John McNesby, president of Fraternal Order of Police Lodge 5, said Rossiter would fight the charges through arbitration.
"He's a good guy and a good detective," McNesby said. "He'll work a case around the clock. He's the aggressive kind of investigator you need to have out there."
Rossiter's three supervisors, who were responsible for verifying his overtime, will be disciplined with suspensions, sources said.
The Police Department's homicide unit has long been known for high overtime costs. In 2009, the department formed an office dedicated to reducing overtime expenses, and city officials have said they cut police overtime costs by more than $15 million.
Rossiter, who joined the department in 1980, has worked some of the city's highest-profile cases in recent years, including the firebombing murders linked to accused drug kingpin Kaboni Savage. He also has been one of the department's highest overtime earners: City records show he has earned more than $90,000 annually in overtime alone in recent years, on top of his salary of around $69,000.
Members of Internal Affairs started investigating Rossiter after getting an anonymous tip, sources said.
"This is the loss of a really experienced, serious detective," said one officer, who spoke on condition of anonymity.
McNesby said the homicide unit functions differently from other branches of the department, particular those officers assigned to overnight shifts, as Rossiter was. The first 48 hours are crucial to solving a murder investigation, and if a detective goes out on a call one hour before he is scheduled to finish the shift, he may work for the next two days straight. Under those circumstances, McNesby said, many detectives would not think twice about going home for a shower and a change of clothes.
Contact Allison Steele at 215-854-2641 or firstname.lastname@example.org. Follow on Twitter @AESteele.