[UPDATE BELOW] An off-duty NYPD detective who was part of a mob of illegal motorcyclists that chased a terrified SUV driver on September 29th witnessed and even participated in the assault, sources tell the Post. It was previously reported that multiple off-duty cops were participating in the ride, but it's been unclear what the cops actually witnessed. Now sources say the officer, a seven-year veteran, was "pounding the vehicle with his fists at the height of the bloody road-rage attack."

Internal Affairs is investigating, and the unnamed detective has been put on modified duty and relieved of his badge and gun. Yesterday a lawyer for the Detectives' Endowment Association told CBS that the officer "did not witness a biker get struck by an SUV or see the SUV driver subsequently attacked by bikers."

The detective waited three days to come forward and inform superiors that he participated in the illegal group ride, which descended into violence on the Henry Hudson Parkway when SUV driver Alexian Lien collided with motorcyclist Christopher Cruz. The detective told investigators he didn’t help Lien because he didn't arrive at the scene until the beating was nearly over. But sources tell the Post that video obtained by investigators clearly shows him banging on one of the vehicle's windows.

CBS News senior correspondent John Miller, a former New York deputy police commissioner, reports that as many as three off-duty NYPD officers participated in the ride, and that one of them is an undercover detective in Internal Affairs. Yes, that's the department responsible for investigating police misconduct. Sources also tell Miller an NYPD sergeant was involved.

"We know of one (officer) who was there at the initial beating," Miller said on CBS, "But these other officers, one of the ones that's come forward says after the guy got run over by the SUV, 'I wasn't there for the rest of it.' ... Here's the problem they're going to face, which is whatever they did or didn't do that day is one issue, but not coming forward right away actually could rise to official misconduct for hindering an investigation once they were aware of it and that's a potential criminal charge."

The lawyer for the Detectives' Endowment Association told CBS the off-duty detective allegedly involved in the assault had no gun or badge with him and "was conscious of cases where officers blew cover only to be suspended or dismissed for doing so." The detective claims he had no choice but to drive away.

A fourth motorcyclist was arrested yesterday at his home in Canarsie. Investigators believe Craig Wright, 29, punched Lien through the shattered driver's side window. Wright is charged with Gang Assault, Assault and Unlawful Imprisonment. More arrests are expected.

Update 12:33 p.m.:DNAinfo's Murray Weiss hears that the off-duty cop who was allegedly involved in the assault on Lien will be arrested imminently. Stay tuned...

Meanwhile Cruz, who was the first biker arrested, gave his first interview to ABC News. With his lawyer seated at his side, Cruz insisted he did not intentionally brake in front of Lien, and was not trying to make him stop. Here's the interview: