Agents collected boxes of tickets and other evidence at the department's traffic unit, where the officers are assigned, police said in a news release.
The FBI is involved because the overtime was paid with federal grant money, police said.
Four to six officers are suspected of deliberately ommitting the time on otherwise legitimate tickets they wrote, then filling in the time later to make it appear they worked overtime, police spokesman Lt. Paul Henderson has said.
Henderson said it is possible even more officers may have been involved.
The investigation started when a supervisor noticed discrepancies in the ticket book of an officer who had gone on leave, Henderson said.
In a written statement Wednesday, Chief Jeffrey Halstead said he knows the vast majority of his officers are honest and work hard to serve the community.
"The intent of this investigation is to uncover any abuse or potential fraud," he said. "The public can rest assured that I will rid this police department of any and every employee that is engaged in criminal misconduct, period.”
Halstead announced earlier he had suspended all overtime under the grant program, which is called the Selective Traffic Enforcement Program, or STEP.
FBI spokesman Mark White confirmed agents have joined the probe at the request of police. He said it could take several months.
Agents from the inspector general's office of the U.S. Department of Transportation also are investigating.