RARITAN — An off-duty Irvington police officer watched two women beat a 17-year-old girl in a Raritan parking lot and then helped the attackers flee, authorities said today.

Herne LaCoste, 40, of Plainfield, was arrested at his home Tuesday in connection with his alleged role in the Sunday morning attack — which netted only the victim’s cell phone, bus ticket and MetroCard, Somerset County Prosecutor Geoffrey Soriano said.

LaCoste has been charged with second-degree official misconduct, unlawful possession of a firearm and possession of a weapon for an unlawful purpose. The 13-year police veteran is one of three people arrested in connection with the crime.

The trio knew the victim before the robbery, but details of their association were not immediately known, said Capt. Jack Bennett, a spokesman for Soriano.

The prosecutor said the victim suffered minor injuries.

In addition to LaCoste, authorities have arrested Shanikah Daniels, 18, of East Orange, and Markeesa Coleman, also 18, of Irvington.

Around 4:30 a.m. Sunday, Daniels allegedly held up the victim at gunpoint inside LaCoste’s gray SUV, which was parked off Route 202, Soriano said. He also said the victim was forced from the SUV, and Daniels and Coleman allegedly beat her as LaCoste stood by.

Afterward, the trio drove off in the SUV with LaCoste behind the wheel, Soriano said.

Later that day, an investigation led county and Raritan authorities to Daniels’ apartment building, and they spotted the gray SUV parked nearby, according to Soriano. Using a search warrant, he said, investigators found the victim’s property inside the apartment and a .38-caliber handgun in the SUV.

Bennett said the handgun was not LaCoste’s police-issued weapon.

Authorities arrested Daniels Sunday, and she faces charges of first-degree robbery, conspiracy to commit robbery, possession of a handgun without a permit and possession of a firearm with the intent to use against another person, Soriano said.

Coleman was arrested Tuesday at a relative’s house, Soriano said. She was charged with third-degree aggravated assault and released on her own recognizance.

LaCoste and Daniels were taken to the Somerset County jail in lieu of $20,000 bail each, according to the prosecutor.

An Irvington police spokesman today declined to comment on the case.

The alleged attack marks the second time in two years that LaCoste has been in the news. In January 2010, he shot and killed a suspected burglar after a struggle in a Newark backyard. At the time, Irvington Police Director Joseph Santiago said the shooting appeared justified.

Township police officials also said after the shooting that LaCoste had several commendations and his internal affairs record showed no incidents that were inconsistent with department rules or regulations.

Anyone with information is asked to contact the Somerset County Prosecutor's Office at (908) 231-7100, Raritan police at (908) 725-6700 or the Somerset Crime Stoppers anonymous tip line at (888) 577-TIPS (8477) or www.888577tips.org.

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