Updated April 21, 2018, with details about Allison Mack's indictment and comments from the U.S. Attorney's Office and the FBI.

Actress Allison Mack, best known for her role as Chloe Sullivan on the Superman prequel Smallville, was charged Friday with sex trafficking for her involvement in a purported women's empowerment group that federal prosecutors say sexually exploited its members.

News 4 New York reports Mack was arrested in Brooklyn, New York, by the FBI, and was arraigned in federal court. She was charged with trafficking, sex trafficking conspiracy and forced labor conspiracy in connection to her role with the group Nxivm (pronounced "Nex-e-um").

Her arrest follows that of group leader Keith Raniere on March 25 in Mexico on multiple charges of sex trafficking and forced labor. Rumblings of the imminent arrest of Mack, characterized as one Raniere's top aides, had circulated since then. Raniere's attorneys maintain his innocence. If convicted, Raniere and Mack each face a mandatory-minimum sentence of 15 years in prison.

The Justice Department describes the Albany, New York-based Nxivm as a pyramid scheme that charges participants thousands of dollars for "inspirational executive coaching" courses, and encourages them to pay for additional courses and recruit more members in order to rise through the ranks of the organization. In 2015, Raniere allegedly started his own secret society within the group, supposedly devoted to female empowerment, in which he coerced women into a master-slave relationship. The federal complaint alleges that Raniere, known as "The Vanguard," had the women branded with a design that incorporated his and Mack's initials. She was identified at the time as an unnamed co-conspirator.

“As alleged in the indictment, Allison Mack recruited women to join what was purported to be a female mentorship group that was, in fact, created and led by Keith Raniere,” U.S. Attorney Richard P. Donoghue said in a statement issued Friday. “The victims were then exploited, both sexually and for their labor, to the defendants’ benefit. This Office and our law enforcement partners are committed to prosecuting predators who victimize others through sex trafficking and forced labor.”

Court documents allege that women recruited into the group, referred to as "DOS" -- an acronym for a Latin phrase that loosely translates to "Lord/Master of the Obedient Female Companions" -- or "The Vow," were coerced into providing "collateral," compromising material such as nude photos or damaging personal information about themselves, family or friends, that would then be released if they ever told anyone about it.

"DOS operated with levels of women 'slaves' headed by 'masters,'" alleges the U.S. Attorney's Office for the Eastern District of New York. "Slaves were expected to recruit slaves of their own (thus becoming masters themselves), who in turn owed service not only to their own masters but also to masters above them in the DOS pyramid. Raniere stood alone at the top of the pyramid. Other than Raniere, all members of DOS were women. Mack is one of the women in the first level of the pyramid immediately below Raniere."

According to court filings, Mack "directly or implicitly" required her slaves to have sex with Raniere. In return, Mack is alleged to have received financial and other benefits from Raniere.

In a statement released on Friday, victim were asked to contact the FBI to help in the investigation of what the agency characterized as an "inconceivable crime."

Raniere had allegedly left the United States in November for Mexico after authorities began interviewing witnesses and victims in connection with Nxivm and the sex club. He was apprehended at a $10,000-a-week villa near Puerto Vallarta, Mexico, where he was said to be living with several women. Reports circulated that Mack was present at the time of Raniere's arrest.

Outside of its Albany base, Nxivm had an active community in Vancouver, British Columbia, where numerous television shows are filmed. Battlestar Galactica actresses Grace Park and Nicki Clyne, and Smallville star Kristin Kreuk were member of the organization at one point. However, Park and Kreuk have since cut ties, with the latter leaving the group in 2012.

Kreuk, who played Lana Lang on Smallville, released a statement last month following Raniere's arrest, clarifying her own involvement in Nxivm, and denying that she was ever part of his "inner circle" or helped to recruit women as "sex slaves."

pic.twitter.com/W0aijK3LcX

— Kristin Kreuk (@MsKristinKreuk) March 29, 2018

Mack, now 35, was reportedly recruited into Nxivm in 2010 by Kreuk.