ALBANY – Actress Allison Mack pleaded guilty Monday to federal racketeering charges connected to her longtime involvement in NXIVM, a cult-like self-help group in the Albany area whose leader is accused of treating women as sex slaves.

Mack, 36, best known for her role as Chloe Sullivan in the CW's "Smallville," signed a plea agreement late Monday in Brooklyn federal court, admitting to racketeering crimes that could put her behind bars for years when she is sentenced in September.

The plea proceeding came the same day jury selection was expected to begin in the trial of NXIVM leader Keith Raniere.

Mack had been facing racketeering, forced labor conspiracy, wire fraud conspiracy and various sex trafficking charges in connection with her role as a high-ranking leader in DOS, a group secretly led by Raniere that purported to be a women's empowerment sorority.

But in actuality, federal prosecutors said the group was essentially a sex ring to benefit Raniere, with women known as "slaves" who reported to "masters" who ultimately reported directly to him and were wholly devoted to his teachings.

Allison Mack accused of recruiting women

Prosecutors say Mack was a co-conspirator, recruiting women to join the group. She reported directly to Raniere.

The women, some of whom were branded with a logo containing Raniere's initials, were presented to him for sex and forced to follow restrictive diets to adhere to his preferred body type, according to prosecutors.

They were also forced to give up collateral – nude photos, statements of family secrets – in order to join the group. DOS leaders threatened to release the information if the women publicly revealed the group's existence, prosecutors claim.

Sean Buckley, Mack's attorney, could not immediately be reached for comment Monday morning.

More:Rochester woman tells all about life with NXIVM's Keith Raniere, her ex

Trial set to begin for remaining defendants

In recent weeks, federal prosecutors have acknowledged in court that Mack and some of her co-defendants have been engaged in plea negotiations.

Nancy Salzman, NXIVM co-founder and president, pleaded guilty last month to one count of racketeering conspiracy, confessing to stealing email addresses and passwords of NXIVM critics and altering a tape used in a lawsuit against one.

Her daughter, Lauren Salzman, also pleaded guilty to racketeering and racketeering conspiracy last month.

With Mack's plea, three defendants remain: Raniere; Seagram's liquor heiress Clare Bronfman, who was a major NXIVM funder; and Kathy Russell, a NXIVM bookkeeper.

Raniere, 58, attended high school in Suffern, Rockland County, before moving to the Albany area to attend the Rensselare Polytechnic Institute.

He gained a devoted following through NXIVM, which taught expensive self-help classes that were based on Raniere's teachings.

But Raniere's critics – including former girlfriend Toni Natalie of the Rochester area, who has spoken out publicly against him for years – have long accused NXIVM of operating like a cult, aggressively suing former members who went on to criticize the group.

Federal prosecutors got involved after Sarah Edmondson, an actress from Vancouver, went public in an account in The New York Times with news that women in DOS, the secret sorority, had been branded on their pubic region with Raniere's initials.

The account also detailed "Dynasty" star Catherine Oxenberg's fight to rescue her daughter from the group.

Plea deal:NXIVM co-founder Nancy Salzman pleads guilty in case of alleged cult

Charges explained:What are NXIVM's Keith Raniere, Allison Mack and Clare Bronfman accused of?

JCAMPBELL1@gannett.com