A Green Bay Police Officer has resigned from the force, accused of falsifying police reports.

Officer Mike Rahn was a 3-year veteran patrol officer who worked the night shift.

Green Bay Police say they are looking at about 500 cases in which Rahn was involved. The police chief says there are only a few cases in which there is significant evidence of wrongdoing.

"Right now, we just have a handful that we have concerns about," said Police Chief Andrew Smith. "But since this officer has been on the job for several years, we'll go back and look at every case that he was involved in to see if there's anything at all that we need to be concerned about in any of his prior arrests."

The department is handling an internal investigation. The Wisconsin Department of Justice - Division of Criminal Investigation and the FBI are also involved.

Here's a timeline of the events leading up to the resignation:

On Feb. 26, Officer Rahn responded to a call and used force. Force incidents are sent to supervisors for review.

On Feb. 28, supervisors reviewed the incident and found the written report from Rahn did not match video from officer's dash cameras, which recorded four angles of the incident.

"So one of our lieutenants from Professional Standards Division took a look at the arrest reports and the narratives from this use of force, and then compared them to the in-car video that we have, that takes video at the scene, and there was some significant anomalies," said Chief Andrew Smith. "What was written didn't match up with what was shown on the video."

"As a result, we decided we were not going to let the officer work in the field anymore."

This discovery prompted supervisors to look at another of Rahn's cases. They found the report did not match the dash camera video. Investigators continued digging into Rahn's cases.

On March 3, police notified the Brown County District Attorney. He told police he would no longer use Rahn in any cases. At that time, the chief pulled Rahn from road duties, assigning him to administrative work in the station, with no contact with the public.

"He [the District Attorney], was putting out notices, official letters to all of the attorneys involved in his cases, advising them he won't be using this officer on any cases," Chief Smith said. "He will not be able to testify."

So far, Green Bay District Attorney David Lasee said they have already dismissed three cases, but there could be more in the future.

"We have been rolling out the letters based on where the case is at in the proceedings. So if it's an upcoming trial or an individual is in custody on a pending case, we are expediting getting the letters out on those cases," said Lasee. "From there the defense attorneys have the opportunity to investigate that, if appropriate in their case."

At the same time, the District Attorney's office is also going over their cases involving Rahn to see if they're at risk.

Lasee said his office has four options: dismiss the pending case, proceed without Rahn as a witness, proceed with Rahn as a witness in cases where evidence corroborates his story or overturn a conviction completely.

On March 22, Police Chief Smith said he had enough evidence to show there was falsifying of police reports and misconduct. Chief Smith pulled Rahn's badge, gun, and identification.

On March 28, Rahn resigned from the Green Bay Police Department.

DCI and FBI agents are looking into possible criminal wrongdoing, which could lead to charges and federal civil rights violations.

Officials say Officer Rahn did not target ethnicity or gender in his arrests.

He handled a variety of cases, including drugs, prostitution, and drunk driving.

Rahn has no use of force complaints from the public or other officers. He has no sustained complaints in his file, meaning he was never found to have committed violations.

Investigators believe Rahn was acting alone, and that no other officers were involved.

Most of the incidents happened within the last six months. At least one dated back to 2014. Investigators are still trying to figure out what caused him falsify the documents.

Police Chief Andrew Smith expressed disappointment and anger. He says these actions can shape the public's opinion about the entire force.

"It's been a difficult time, and you know, it's been a difficult time for officers that are out there, because you know, a huge majority of officers are out there doing the right thing every day," said Chief Smith. "And when somebody comes along and does something terrible like this, and it's a despicable thing that someone's done here, it taints everybody. It tarnishes the badge and makes our job that much more dangerous and that much more difficult."

Action 2 News' Sarah Thomsen will have full reports and reaction from Police Chief Andrew Smith starting on Action 2 News at 4.