An undercover police detective participating in a motorcycle rally that descended into violence pounded on the rear window of an SUV while other bikers attacked the driver through his window, according to a criminal complaint Wednesday charging him with gang assault.

Wojciech Braszczok was an “active participant” in the attack on Alexian Lien’s car two weeks ago following a chase up Manhattan’s West Side Highway, prosecutors said. He did nothing to stop it, didn’t report it to his superiors for two days, and then when did, he lied about it, changing his story to say he saw the attack but didn’t participate, said Assistant District Attorney Samantha Turino.

“That is clearly not the case,” Turino said. Law enforcement officials have said video from the scene shows the undercover officer participating.

The SUV driver was eventually pulled from his car and beaten on the street, police have said, though the undercover officer is not charged with attacking Lien.

Braszczok, 32, who appeared in court sporting a beard and wearing a hooded sweatshirt and camouflage pants, was also charged with assault and criminal mischief. He was off-duty during the rally.

His lawyer, John Arlia, said the video actually exculpates his client, who was at least 12 feet from the attack on Lien, a married father of a toddler whose family was in the car at the time.

“This particular video in question exonerates my client — it is an absolute overcharge,” he said. The SUV’s back window already had a “gaping, softball-sized hole” in it, he said.

The detective posted $150,000 bail and left the courthouse with his hood pulled up as far as possible. He climbed into a waiting car.

Braszczok, who worked in the intelligence division, was stripped of his gun and badge after internal affairs detectives discovered he was present for at least part of the melee during a periodic motorcycle rally. Police say it was touched off when a motorcyclist and Lien’s SUV bumped.

Video captured at the scene shows riders swarming around the SUV before the driver takes off, crushing a rider beneath the car. Biker Edwin Mieses Jr. of Lawrence, Mass., broke his legs and suffered spine injuries that his family has said will leave him paralyzed.

Lien’s wife has said he fled to protect her and their 2-year-old daughter. Lien hasn’t been charged with any crime.

The detective initially told authorities and his union that he left without intervening, in part because of his role as an undercover. Uniformed officers, even off-duty, are expected to jump in if a crime is occurring, but rules are murkier for undercover officers who face blowing their covers.

But investigators discovered video evidence showing Braszczok punching an already damaged back window, causing it to shatter, then twice kicking the passenger side of the SUV before leaving the scene, prosecutors said.

The arrest added to the complexities of the Sept. 29 episode, which authorities say began with a reckless motorcycle group ride. More than 1,000 bikers participated.

Several bikers have been charged criminally, either with attacking Lien or his car or instigating the violent encounter.

Clint Caldwell, 32, was also charged Wednesday with gang assault after prosecutors said he pulled out the shattered glass from Lien’s window and reached inside the SUV. His lawyer, Raymond Colon, says there’s no video that shows him with his hands inside the car.

(AP)