"Motorcycle Escapades And New York’s Finest"

Cops as criminals and criminals as cops. How to tell the difference?

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.