New York, NY —

The NYPD and the Manhattan district attorney are investigating the circumstances surrounding an NYPD officer knocking a cyclist to the ground last Friday in Times Square.

The incident, part of a Critical Mass bike ride, was captured on video and surfaced on the Internet.

Kelly said after seeing the video on YouTube, he couldn't rationalize the officer's actions.

KELLY: Well, I have no explanation. I can't explain why it happened.

REPORTER: He says the officer has since turned in his badge and gun and is on modified duty.

Kelly said the department will interview witnesses and police supervisors. The cyclist in the video was arrested that night and charged with attempted assault and resisting arrest.

Tensions between Critical Mass, a pro-bike group, and the NYPD have been longstanding. In 2004 during the Republican National Convention the police arrested hundreds of Critical Mass cyclists for parading without a permit.