A self-help guru groomed women for sex, subjecting them to "shame and humiliation" and threatening to expose their "deepest, darkest secrets" if they didn't comply with his wishes, a federal prosecutor said on Tuesday.

Assistant U.S. Attorney Tanya Hajjar presented opening statements at the sex-trafficking trial of Keith Raniere, the former leader of an upstate New York group called NXIVM that's been likened to a cult.

The prosecutor alleged that Raniere had sex with a 15-year-old girl and took nude photos of her; she also described how some female followers were branded with Raniere's initials.

Authorities have said the branding was done using a cautery pen without anesthesia by a doctor who is now under investigation by state health officials. Eight "Jane Does" in that case have refused to answer questions, saying through their lawyers "the branding was a voluntary free expression of personal beliefs."

Keith Raniere, founder of NXIVM, entering a Dalai Lama event at the Palace Theater in downtown Albany on May 6, 2009. Patrick Dodson

But Raniere's trial is expected to feature testimony from women who claim they were forced to have sex with Raniere. Five of Raniere's co-defendants, including TV actress Allison Mack , have pleaded guilty

Raniere denies criminal wrongdoing; his lawyers say his relationships were consensual.

In court papers, defense lawyers have said the alleged victims were never abused. The women were instead described as "independent, smart, curious adults" in search of "happiness, fulfillment and meaning."

But legions of NXIVM defectors and detractors have called the self-help rhetoric a brainwashing device that has destroyed lives.

Promotional material for the now-disbanded NXIVM once hailed Raniere as a "scientist, mathematician, philosopher, entrepreneur, educator, inventor and author" who has "devoted his life to developing new tools for human empowerment, expression and ethics."

"The defendant pretended to be a guru," Hajjar said, "but he was a criminal."