An off-duty undercover cop who claimed he took no active role as fellow bikers pulled a Manhattan dad from his SUV and beat him to a pulp actually furiously slapped the car’s back window so hard that it shattered at the height of the bloody road-rage attack, sources told The Post.

The cop, a seven-year veteran, had told investigators he didn’t help the injured man because he rode up to the scene as the beating was nearly over, sources said.

The development came as another biker who helped in driver Alexian Lien’s beat-down was arraigned in Manhattan Court on Tuesday.

Craig Wright, 29, of Brooklyn can allegedly be seen on video throwing punches through the shattered driver’s-side window of the 33-year-old victim’s Range Rover in front of Lien’s terrified wife and 2-year-old daughter. Wright is also seen allegedly kicking Lien outside the SUV. He was held on $100,000 bail Tuesday.

As for the undercover cop, disgusted law-enforcement sources said Monday that Internal Affairs Bureau higher-ups want to nail the officer.

“The cop is a total mope. F—ing embarrassment, ’’ one source seethed. “They should can him immediately. Anyone else who was even peripherally involved should be canned.”

Still, probers believe their hands are tied, sources said, because prosecutors declined to press ahead with the case against another biker, Allen Edwards, 43, of Queens, who allegedly punched the driver’s side rear window of Lien’s SUV before Lien was pummeled.

At the time, the chief of the Manhattan district attorney’s Trials Division, Karen Friedman Agnifilo, defended the move, saying, “Prematurely charging individuals with low-level crimes does not further the goals of the investigation and could weaken the cases we expect to bring against the perpetrators of serious crimes.”

The investigation is continuing, a source noted.

But the undercover cop will likely face internal charges. He didn’t tell his bosses he was with the bikers — much less at the crime scene — until more than three days after the Sept. 29 attack.

The unidentified cop has already been placed on modified duty and turned in his gun and badge.

“I think we all, no matter what your job is, have an obligation to help one another,’’ Mayor Bloomberg said on MSNBC’s “Morning Joe’’ on Monday, in response to questioning about the officer’s actions. “And if you see somebody getting beaten up, you know, let’s go jump in and stop the fight.”

Cops are searching for at least two more suspects.

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.”

Additional reporting by Kirstan Conley, Rebecca Rosenberg and Yoav Gonen