Television actress Allison Mack and NXIVM founder Keith Raniere have been indicted on federal charges of sex trafficking and conspiracy to commit forced labor for their alleged roles in a secret sex-slave club in which women were also branded with the pair's initials.

The Times Union first reported last week that Mack, 35, was identified in FBI records filed in support of a recent search warrant application as an alleged co-conspirator with Raniere in his efforts to coerce women into joining the secret club.

Federal prosecutors said Mack was arrested in New York City early Friday. She was arraigned by a U.S. magistrate judge late Friday afternoon and ordered held in custody of U.S. marshals pending a bail hearing scheduled for Monday in Brooklyn. Raniere has been held in custody without bond since his arrest last month and will be arraigned on the indictment at a later date, they said.

The indictment, which was filed Thursday and unsealed on Friday, charges both Raniere and Mack with multiple counts of sex trafficking, conspiracy to commit sex trafficking and conspiracy to commit forced labor. They each face a potential minimum sentence of 15 years in prison if convicted.

FBI records say Mack helped recruit women into the club and made them pose for nude photographs that she would forward to Raniere. She also delivered some of the women to Raniere for sexual encounters, and at least one alleged victim described the sex as unwanted.

"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,” said U.S. Attorney Richard P. Donoghue of the Eastern District of New York in Brooklyn. “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.”

Mack, an actress who is best known for her role in the television series "Smallville," which ended in 2011, had initially been listed in a federal criminal complaint as Raniere's unidentified co-conspirator.

According to people familiar with NXIVM, Mack joined the organization about a decade ago, around the same time several other young actresses were pulled into its ranks, including Nicki Clyne, Grace Park and Kristin Kreuk, who recruited Mack. Kreuk left NXIVM several years ago — before the slave-sex club was allegedly formed by Raniere — and has denounced the practices made public recently.

Mack was with Raniere in Mexico when he was taken into custody on March 25 by Mexican federal police officers, who arrested Raniere at the request of U.S. law enforcement authorities.

Raniere, whose organization has been described by experts as a cult, was arrested at a luxury villa in Puerto Vallarta.

The federal complaint alleges that Raniere organized the secret group within NXIVM in which women said they were coerced into joining a slave-master club and later branded with a design that included the initials of Raniere and Mack.

Raniere, in statements previously posted on NXIVM's website, had characterized the slave-master group as a consenting, private "sorority" and said that he and the corporation had no role in it.

But federal court records indicate emails seized from Raniere's private messaging accounts "support the conclusion that Raniere created" the club, which was known as "Dominus Obsequious Sororium," which means "Master Over the Slave Women."

The women in the group, according to the federal complaint, were lured into the club by other female NXIVM members, including Mack, who considered Raniere her "grand master," and were required to provide "collateral" in order to join. If they tried to leave, they were threatened that their collateral —sometimes damaging information about family members or close-up photographs of their genitalia — would be released.

In the federal criminal complaint filed last month against Raniere, prosecutors said a Halfmoon residence known as the Library was where Raniere, 57, repeatedly had sex with an unidentified actress in her 30s.

That townhouse on Hale Drive, which was searched by the FBI on March 27, is where federal authorities say the unidentified actress, who is listed as "Jane Doe 1" in the criminal complaint, became a victim of sex trafficking and forced labor. The actress lived in Brooklyn during the time she was allegedly a victim of Raniere's — which is part of the reason the case is being pursued in Brooklyn.

During one of her trips to Halfmoon, the actress said Mack instructed her to meet Raniere in the middle of the night near his residence. He allegedly instructed the actress to remove her clothing, placed a blindfold over her eyes and drove her around. Once he stopped the vehicle, they walked through some woods to a shack where the actress said she was tied to a table and forced to engage in oral sex with an unidentified person who was in the room with Raniere. It's unclear if the unidentified person was male or female.

The state Health Department initially brushed off the allegations that women in the secret club were being branded by a doctor, Danielle Roberts, whose license to practice medicine in New York remains intact. After reports about the branding became public, the stage agency reopened its examination of the allegations against Roberts, who two women said used a cauterizing tool to brand multiple participants in the secret women's group.

According to information referenced in a complaint filed with the state by Sarah Edmondson of Vancouver, at least 20 women associated with NXIVM were lured into the club and required to provide "collateral."

Edmondson and another woman involved with NXIVM told the Times Union that they were brought into the club and subsequently branded by Roberts, who then gave them special bandages and advice on helping the wounds heal.

Edmondson said she was told it would be a "tattoo" and that the symbol was a Latin symbol for "the elements." She learned weeks later that the brand was actually the initials of Raniere and Mack, whom Edmondson's complaint identified as having "started" the secret women's group with Raniere.

State Health Department officials last year said they were researching what regulations there may be, if any, governing human branding. In July, they had dismissed Edmondson's complaint, saying the allegations "did not occur with the doctor-patient relationship and should be reported to law enforcement."

Edmondson said she also contacted State Police, but an investigator told her there was no criminal conduct because the women agreed to be branded.

The U.S. attorney's office in Brooklyn opened its investigation last fall.

Federal search warrant records that were unsealed two weeks ago in response to a request by the Times Union said Raniere has received financial support from Clare W. Bronfman, an heiress of the Seagram Co. business empire who has described herself as the operations director of NXIVM. The records indicate Bronfman also assisted Raniere in sending threatening letters purported to be from a Mexican attorney to women who had defected from the secret club or criticized NXIVM.

"Additionally, the heiress has made multiple attempts to have criminal charges brought against a former DOS slave, who has discussed her experience in the media," the search warrant documents state.

Raniere fled to Mexico last fall with Bronfman, a member of NXIVM's executive board who has supplied Raniere with access to millions of dollars and private jets, the records indicate.

"Prior to this trip, Raniere had not flown out of the country since 2015, when he visited the heiress' private island in Fiji," according to the FBI.

In Mexico, authorities said, Raniere got rid of his mobile phone and used only encrypted email to communicate with his followers. It took authorities nearly two months to locate and arrest him in Mexico.

The search warrant records indicate there are other federal crimes being investigated. Sources close to NXIVM have previously alleged, in court records, that the organization was involved in kidnapping, money laundering, tax evasion and human trafficking.

Note: An earlier version of this story gave an incorrect year for the end of the series "Smallville."