Sources tell Action News the investigation is focused on an expensive scope attached to a high powered rifle that went missing from the Chester Police Department's evidence room in March.
Delaware County District Attorney's Office began its investigation after a Chester officer made a formal complaint.
On Tuesday afternoon, detectives served a search warrant at the police station and expanded its investigation.
The scope turned up shortly after it was reported missing, but who returned it and how is unclear.
The DA's office confirmed there is an investigation surrounding the evidence room, but wouldn't comment further.
"We've asked our police department to cooperate with them fully. We want this investigation, whatever it is, to occur so we can get past it and give them any information pertaining to what they're seeking," Chester Mayor John Linder said.
Sources say investigators are questioning officers and working to review surveillance video of the evidence room.
They also say the integrity of the evidence room is part of the expanding probe.
Only a couple of officers have access to the room.
At this point, the investigation is focused solely on the missing scope.
Sources emphasize there is no indication other evidence went missing, but detectives will go where the investigation leads them.
Chester Police Commissioner Joseph Bail was out of town today and told Action News he had no comment.
"I have spoken to the commissioner and informed him about the warrant being served and we will talk later about what the process entails," Linder said.
As of now, no officer has been accused of crime nor has any disciplinary action been taken.