ALBANY — A suspected prostitute and a 15-year-old girl were guests of honor at a sex party for gang bangers, a pair of convicted killers and other inmates serving time at an upstate prison for teens, The Post has learned.

The “social” at the Goshen Secure Center in Orange County on Dec. 12, 2009 — caught by security cameras — included a “lap dance,” suspected oral sex and other possible sexual acts, sources said.

Three agencies are probing the shocking orgy.

“There was a girl getting her pants pulled down and getting ready to be penetrated from the rear,” said Antonio Collado Jr., 58, a security officer at the facility for offenders under 19.

“I saw her red g-string panties almost being pulled off.

“The person in the control room who was controlling the security camera said this had occurred after the resident had already received oral sex. And this girl was a minor.”

He said the dance was abruptly cancelled once the officer monitoring the cameras raised the alarm. Collado was among the guards called to the scene.

A source said the tape shows “a lap dance going on, and the question is whether it shows one or more sex acts.”

Collado has been granted legal “whistleblower” status by investigators.

“I’ve seen socials before but have never seen anything like this,” said Collado, who has worked for the state Office of Children and Family Services and its predecessor, the Division for Youth, for 38 years.

Collado, whose agency houses convicted criminals considered too young to be immediately placed in state prisons, said he was ordered to transport two of the women invited to the dance in a state car from “a street corner” in Albany to Goshen and then back again — a nearly four-hour round trip.

Both he and an investigative source said only superficial background checks were conducted on the two females before they were taken to the prison.

They said one was just 15 and the other “a suspected prostitute” who had received at least $100 in the mail from one of the prisoners.

The prisoners were allowed to select “the girls they wanted to have attend,” said Collado.

Collado said one inmate later told him he’d never met the woman he invited to the dance but had gotten her name from a fellow prisoner.

“An investigation is being conducted concerning a social held at Goshen because it clearly appears that inappropriate conduct, including a lap dance and something else, took place,” State Inspector General Joseph Fisch told The Post.

OCFS and the Department of Correctional Services are also conducting investigations, it was learned.

OCFS spokeswoman Susan Steele refused to discuss details of the Goshen incident, saying only: “An investigation is going on now.”

She said OCFS Commissioner Gladys Carrion supported social dances for hard-core convicted youth “as part of our way of incentivizing good behavior.”

Fisch’s probe was sparked by a tip from state Sen. Catharine Young (R-Olean), who is investigating increasing levels of violence against prisoners and staff in OCFS-operated facilities.

Young paid an unannounced visit to a “social” at the Brookwood Secure Center for youths in Columbia County last summer.

She was “shocked” by what she saw, including several youths convicted of serious violent crimes being allowed to bring young women into the facility, she said.

“What if one of the residents get into a fight over one of the women?” Young said.

fredric.dicker@nypost.com

