The officer did not step in to stop the assault, and the New York Police Department internal affairs unit has launched an investigation to determine why he did not report his presence at the beating until Wednesday, CNN reported Saturday.
The disclosure came as two more bikers linked to the assault, Reginald Chance and Robert Sims, turned themselves in.
Chance is seen in a video banging his helmet against the sport utility vehicle driven by Alexian Lien, police said. Sims can also been seen pursuing the vehicle.
The video was taken by a camera attached to the helmet of one of the riders in the group.
Four of the bikers, who had allegedly surrounded Lien's SUV, were injured when he drove his Range Rover into the group in an attempt to escape.
Rider Christopher Cruz has been charged with reckless driving and other misdemeanors in connection with the incident. Cruz, 28, was slightly injured after he slowed quickly in front of Lien's vehicle, causing the vehicle to bump his rear tire.
Another biker was released from custody Wednesday after police determined he might have been trying to help Lien.