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The spiritual leader of an alleged New York sex cult was hit with child pornography charges Wednesday, escalating a case that already featured allegations of female "slaves" forced into having his initials branded onto their flesh.

The additional charges against Keith Raniere, 58, were revealed hours after Nancy Salzman, the co-founder of the Albany-based group known as NXIVM, pleaded guilty to a charge of racketeering conspiracy.

Federal prosecutors said in court papers that Raniere, who was known inside the organization as "Vanguard," engaged in relationships with two underage girls, including a 15-year-old.

The government has images of the 15-year-old, "constituting child pornography, that were created and possessed by Raniere and electronic communications between the victim and Raniere reflecting their sexual relationship and indicating that it began when she was fifteen years old," prosecutors from the U.S. Attorney's Office in Brooklyn said in court papers filed Tuesday.

Raniere's lawyer, Marc Agnifilo, disputed the new charges in a sharply worded statement.

"These eleventh hour charges three weeks before the trial begins serve only to taint the jury panel," Agnifilo said. "Had they been legitimate, the government would have brought these charges a year ago."

Earlier Wednesday, Salzman, 65, admitted to committing racketeering offenses, including stealing the identities of some critics of the group, while working as the president of NXIVM.

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Nancy Salzman exits court following a hearing at the United States Federal Courthouse in Brooklyn on July 25, 2018. Brendan McDermid / Reuters file

Salzman, who was known to her followers as "Prefect," faces 33 to 41 months in prison. Her sentencing is set for July 10.

"I did things I knew were wrong and justified it was for the greater good," Salzman said in a hushed voice during a hearing at Brooklyn Federal Court. "Some of what I did was not just wrong but criminal. If I could do it all over, I would, but I cannot."

Salzman, Raniere and four other members, including "Smallville" actress Allison Mack and Seagram's liquor heiress Clare Bronfman, were indicted last July for their roles in running the controversial group. Salzman's 43-year-old daughter, Lauren, was also ensnared in the case.

Allison Mack is escorted away from federal court in Brooklyn on Tuesday. Frank Franklin II / AP

Supporters say it was a self-help group committed to changing the world. Prosecutors describe it as a criminal enterprise built around a pyramid scheme designed to enrich the top officials and supply the leader with a stable of sex "slaves."

Members paid thousands of dollars for NXIVM-sponsored classes promising personal and professional development. Prosecutors say the courses forced many into debt, drawing them into a multilevel marketing scheme that rewarded the recruitment of others with payments and increased status.

The group was led by Raniere, a self-described ethicist who prosecutors say used the organization to satisfy his sexual appetite.

Raniere is accused of creating a secret society within NXIVM that coerced women into having sex with him and having his initials branded on the skin below their hips.

The secret group was called DOS, an acronym for "Dominus Obsequious Sororium," which translates to "Lord/Master of the Obedient Female Companions," according to court papers.

Prosecutors say DOS masters groomed their slaves for sex with Raniere and forced them to turn over "collateral" — sexually explicit photos and damaging secrets — that would be made public if they ever disclosed the existence of the secret society.

Raniere denies that NXIVM was a cult or pyramid scheme and says any sexual relationships were consensual. He has pleaded not guilty to racketeering, trafficking and conspiracy charges.

Mack, 36, who prosecutors say recruited "slaves" for Raniere, pleaded not guilty to several charges including sex trafficking conspiracy, racketeering conspiracy and wire fraud.

Bronfman, 40, who prosecutors say helped bankroll the organization and is paying the legal fees for her co-defendants, pleaded not guilty to racketeering conspiracy, conspiracy to commit identity theft and other charges.

A trial is scheduled for mid-April.