An off-duty undercover NYPD cop was among the pack of bikers who chased a family up the West Side Highway — and he stood by as the dad was hauled from his car and beaten, sources told The Post.

The unidentified officer waited three days to come forward Wednesday night. He has been placed on modified duty and turned in his gun and badge on Friday, the sources said.

He rides with the New Rochelle-based Front Line Soldiers, a club that also counts several other cops among its members, a source said.

Internal Affairs is investigating whether those cops, too, were among the bikers — and whether they also witnessed the assault that left Internet exec Alexian Lien bleeding on the pavement as his wife and toddler cowered in their black Range Rover.

“It does not appear that he got involved at the scene,” one law-enforcement source said of the undercover, who has hired a lawyer. “He didn’t want to blow his cover,” said the source — though the cop was not investigating the group of bikers.

Cops have uncovered new photo and video evidence showing that Lien was attacked by as many as five bikers, sources said.

One of those bikers was allegedly Robert Sims, 35, of Brooklyn. He was taken into custody Friday afternoon and charged with gang assault, assault, criminal possession of a weapon, attempted assault and attempted gang assault, a source said.

“Apparently he hit the guy,” the source said.

Reginald Chance, 38, of Brooklyn — depicted in video using his chrome-colored helmet to bash in Lien’s driver’s-side window — was also in custody Friday awaiting charges. Police plan to do lineups Saturday morning.

There is no evidence that he beat or slashed Lien, but Chance faces possible charges of gang assault, a source said.

Several other bikers at the center of the beatdown were also wearing chrome-colored helmets, hampering the investigation into Chance, a source said.

But the new stills and footage — which surfaced from non-biker Samaritans who pulled over to help Lien — have helped with the investigation.

Prosecutors re-interviewed the Liens on Friday afternoon and also planned on taking a closer look at the black Range Rover that was pursued from 125th Street up to 178th Street.

The battered vehicle was moved via flatbed Friday afternoon from a police lot to the 33rd Precinct in Washington Heights, where the investigation is based.

The Range Rover’s smashed windows are a record of the vehemence with which the berserk bikers tried to get at Lien, whose window was busted clean out.

Wife Rosalyn’s window was fully spider-webbed, and the rear window above the tailgate was also broken out completely.

The length of time it’s taken to start throwing thug bikers in handcuffs has irritated some investigators, who fault prosecutors for what they characterize as the DA’s methodical, wait-for-felony-evidence approach.

“You should slap them with the charges that you can prove, whether they’re lesser or more serious charges,” said one cop. “Later on, you can always upgrade or downgrade the charges.”

Besides Sims only one other biker has been charged in the rampage despite videos, still photos and non-biker witness accounts from the broad-daylight attack.

Christopher Cruz, 28, of Passaic, NJ, faces misdemeanor charges of reckless driving and imprisonment for suddenly slowing his bike in front of the Range Rover, causing the fender-bender blamed for sparking the chase and violence.

Meanwhile, the only biker injured during the rampage threatened he may sue Lien for running him over with the Range Rover when he hit the gas as his SUV was first surrounded.

“He was attempting to diffuse the situation,” lawyer Gloria Allred said of Edwin Mieses, 32, whom she said may be paralyzed from the waist down “for the rest of his life.”

Mieses hasn’t had a valid license in his home state of Massachusetts since 1999, and haas never applied for a motorcycle license.

Video by Kevin Fasick

Additional reporting by Kevin Fasick, Lia Eustachewich and Jennifer Bain