New York Post obtained a video that showed he slapped the victim’s car window so hard that it broke. Braszczok, a 10-year veteran of the job was initially charged with criminal mischief, but on Wednesday prosecutors added a felony charge of gang assault. Braszczok was undercover during the Occupy Wall Street bust last year, and even attended meetings and lived with the protesters.

The motorcyclists chased Lien and eventually started attacking his car with his wife and baby daughter inside the vehicle. Lien was dragged out and then beaten and stomped on by at least six motorcyclists. A good Samaritan intervened, but motorcyclists also slammed the back seat windows, where the baby was sitting.

When Braszczok disclosed he had been at the race, he claimed he only witnessed the attack while on the off-duty ride. The AP reports, "But investigators discovered video evidence showing him punching an already damaged back window, then twice kicking the side of the SUV before leaving the scene, according to two people familiar with the case. The people weren’t authorized to discuss the inquiry and spoke to The Associated Press on the condition of anonymity." NYPD internal affairs investigators had initially been looking into the undercover detective’s conduct because he didn’t report until three days later that he had been at the rally.

As far as the unreleased video showing Braszczok "punching an already damaged back window," , note that among the various sources reporting, that even this detail remains unclear.

According to the Post, "The cop also said he feared for his life if he blew his cover in any way — but law-enforcement sources scoffed at the claim. He works undercover narcotics, but is on assignment with the NYPD’s Intelligence Division and wasn’t on the clock at the time." A source said, "He totally lied when he said he did nothing because he didn’t want to blow his cover." The Post adds, "If he caused $1,500 or more in damage to Lien’s car, Braszczok faces a D felony, which carries up to seven years. The Range Rover’s rear window costs $1,730."

The NYPD's Internal Affairs unit hopes the officer will be held responsible for the attack, although last week's decision by the District Attorney to drop charges against Allen Edwards, who was also seen punching Lien's window, might prevent formal charges from being filed. However, the officer will likely face internal discipline -- he's already been placed on modified duty -- which required him to turn in his gun and badge.

The officer is reportedly one of six NYPD cops who were riding with the motorcycle group during or before the attack; two detectives, one sergeant, and three police officers are being investigated, according to DNA Info. None of the police officers intervened in the beating.

Meanwhile, Christopher Cruz, 28, who allegedly started the chain of events by slowing down his bike to let his pals ahead -- leaving Lien to bump his car into him -- said in an interview aired Monday that he felt no responsibility.

“I was looking over my shoulder to see where my friends were,” he told ABC News. “I wanted them to pull in front so I could follow them. I didn’t brake, but when I looked over my shoulder, my hand came off the throttle a little, but the driver didn’t slow down at all and bumped me.”

The investigation is continuing.