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TODAY   |  October 09, 2013

Undercover cop smashed window in NYC biker attack

New video released only to police officials that shows an undercover detective’s involvement in an incident involving an SUV and motorcyclists, showing him smashing a rear window of the SUV. NBC’s Stephanie Gosk reports.

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>>> this morning in the case of the motorcycle gang that attacked a man driving an suv here in new york . this time it's an undercover police officer facing charges. stephanie gosk has details on that. good morning to you.

>> reporter: good morning, this undercover detective came forward three days after the incident but law enforcement officials tell nbc news that he first told them he never got involved in the altercation but when new video surfaced showing his actual role in the attack, sources say police officials felt duped. the bikers had no idea as the confrontation between them grew more intense and more violent a new york city undercover detective was there the whole time. moments after a biker attacked alexian lien's driver's side window with his helmet, new video shows that the detective joined in and smashed the suv's rear window . police sources say he took off on his bike and lien was pulled from the car and did not take part in the beating. the 32-year-old police officer faces charges of riot and criminal mischief. his undercover operation that the nypd describes as deeply sensitive is over. even before the new video was uncovered, the head of the new york detectives union had concerns that it took three days for the detective to come forward and admit he was there.

>> why did it take him so long to step up?

>> well, i don't know that but that's going to be problematic.

>> reporter: internal affairs was already investigating five other off duty police officers that may have taken part in the unauthorized rally. the story that began with a dramatic confrontation caught on video has now unexpectedly shined a light on law enforcement 's most sensitive operations. an undercover detective that investigates biker gangs in florida spoke to nbc news.

>> if you're infuwith a group in deep cover you're living with them day-to-day.

>> reporter: for an undercover cop there's always the danger of being exposed.

>> well, there have been federal offices as well as state that have been killed when their identities have been compromised. so it with can rather dangerous.

>> reporter: the new york detective turned himself in as police released photos of four more suspects wanted for questioning. this investigation is far from over. sources at the nypd say this detective had been involved in occupy wall street operations but now his identity is exposed and badge and gun turned in and he faces years in prison, matt.