He served and protected Who/What?

An off-duty undercover New York City police officer was among the crowd of motorcyclists on the scene as SUV driver Alexian Lien was beaten on Sunday, officials confirmed to ABC News today.

The undercover narcotics cop did not intervene in the beating out of fear that his cover would be blown, sources said.

The cop's identity has not been released by the NYPD. According to officials, the officer notified his superiors days after the incident. He was a motorcyclist who was there on his own time, they said.

The information is part of the ongoing investigation by the NYPD and the Manhattan district attorney into the confrontation and beating that happened Sunday afternoon between a group of 20 to 30 motorcyclists and Lien, caught on video and uploaded to the Internet.

The officer's involvement is also the subject of an Internal Affairs investigation, sources said.

Lien was driving his SUV on the West Side Highway in Manhattan with his wife and 2-year-old as part of a wedding anniversary celebration around 2 p.m. Sunday when their SUV was surrounded by the motorcyclists, who were doing an annual ride through the city, police and Lien's wife said.

The bikers tried to slow traffic and one biker cut Lien off, slowing down and leading Lien to bump into the biker, police said. The group of motorcyclists then slowed down further and surrounded the SUV, causing Lien to accelerate away from the group. Lien's wife, Rosalyn Ng, said that her husband feared for their lives when he decided to take off to get away from the group. As he sped off, Lien hit some of the motorcyclists, critically injuring one, authorities said.

Other motorcyclists then chased Lien's SUV and then bashed in his windows, pulled him out of the car, and beat him, police said.

The undercover police officer was at the scene where Lien was beaten, officials said.

The prime suspect of the beating is still at large, though he could surrender to cops as early as this evening, officials said.

A source briefed on the case told ABC News that in the days since the attack "new evidence has been coming in and it will give prosecutors much better evidence for charging the attackers."

The new evidence includes additional video footage and stills compiled from a variety of cameras that were situated around the scenes of the chase and melee, the source said.