By Larry Celona, Jamie Schram and Laura Italiano

October 4, 2013 | 3:19pm | NY Post

An off-duty, NYPD undercover officer was among the pack of motorcyclists who chased a Financial District family up the West Side Highway — and he stood by as the dad was hauled from his car and beaten, The Post has learned.

The unidentified officer only came forward Wednesday night, four days after the caught-on-video biker predation, and is described as a motorcycle hobbyist who rides with Front Line Soldiers, a New Rochelle-based group 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 any of them are also witnesses to the beating that left Internet exec Alexian Lien bleeding on the pavement as his wife and toddler daughter cowered inside their black Range Rover.

“It is does not appear that he got involved at the scene,” one law enforcement source said of the undercover.

“He didn’t want to blow his cover,” said a source — though he was not investigating those riders.

The undercover has now lawyered up, the source added.

Meanwhile, sources said that cops have developed new photographic evidence showing Lien was physically attacked by as many as five bikers.

None of those bikers are under arrest six days after the incident and only one has been identified by name, one source said.

There is as yet no evidence that suspect biker Reginald Chance, 38, of Brooklyn — depicted in video using his chrome-colored helmet to bash in Lien’s driver’s side window — was among those who beat or slashed Lien, a source said.

Chance, who has also lawyered up, still faces possible charges of gang assault, a source said.

The investigation into the incident is amping up Friday, with prosecutors set to re-interview the couple this afternoon.

Police are also taking a closer look at the black Range Rover that became a rolling prison for the family as it was pursued from 125th Street up to 178th Street.

The battered vehicle was moved via NYPD flatbed this afternoon from a police lot to the 33rd Precinct, where the police investigation is based.

The 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 Ng’s passenger side window was fully spider-webbed, and the rear window above the tailgate was also broken out completely.

Still, 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,” complained one cop. “Later on, you can always upgrade or downgrade the charges,” he said.

“What’s the difference? The DA’s Office might be playing this too cautiously. They [the bikers] were coming at this family from all angles.

“At the minimum, they have the perps on reckless endangerment because they smashed the windows. Flying glass could have injured the family — especially the child.”

As of Friday morning, only one biker had been arrested despite the amount of video and still evidence and the number of non-biker witnesses to the broad daylight mayhem.

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.