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Facing overwhelming evidence, an Eagan man admitted Thursday to masterminding a yearslong "sextortion" scheme that prosecutors say victimized at least 178 high school boys in Minnesota and other states.

Anton Martynenko, 31, pleaded guilty in U.S. District Court in St. Paul to three child pornography counts: production, distribution and advertising.

At least twice, prosecutors say, Martynenko's ruse also led to sexual activity with underage boys.

According to his plea agreement, Martynenko posed as a young female on various "decoy" social media accounts and solicited nude images and videos from high school boys, from as early as 2011 through November 2015. He kept folders of nude photos and videos of young women he posed as, and would swap those with his male victims.

Assistant U.S. Attorney Carol Kayser said Thursday that most similar cases involve two to six victims on average, not the "at least 20" Martynenko admitted to in court or the 178 cited by the government. Martynenko distributed victims' nude photos more than 1,000 times, according to his plea agreement.

"People don't think boys can be victimized," Kayser said.

In October, investigators seized electronic devices from Martynenko's home, including a thumb drive with folders of nude images and videos sorted by name and age — and sometimes naming their high school. Those still investigating the case have said the total number of victims could tally more than 400 people across Minnesota, Wisconsin and Illinois.

It was also revealed Thursday that Martynenko, using a decoy social media profile, persuaded at least two boys on separate occasions last year to come to his home and let him perform oral sex on them — promising one that the girl he was posing as would have sex with him in return and threatening the other that his nude photos would appear online if he didn't comply.

Kayser said Hennepin and Dakota County prosecutors agreed not to pursue criminal sexual conduct charges as part of Martynenko's plea in this federal case.

Martynenko will be sentenced at a date to be determined and faces a mandatory minimum of 15 years in prison. The charges carry the possibility of an 80-year sentence altogether and $250,000 fines for each count.

During the period under investigation, Martynenko spent most of his time at his Eagan home, which is owned by his mother, at work, or going to sporting events, according to court documents.

While working at a mortgage lending firm in Eagan, Martynenko regularly exercised over lunch breaks at a Life Time Fitness in Lakeville, even though there was a gym from the same chain much closer to his office, according to a search warrant. Local high school athletes also frequented the Lakeville gym, and a thumb drive taken from Martynenko's home contained a movie he shot of males changing in a locker room.

As investigators prepared to raid his home in October, they believe Martynenko tried to destroy a computer they later found doused in water and hidden in a laundry room, according to documents filed in court. By the time they finished their sweep, authorities had enough evidence for an insurmountable case.

Martynenko chose to plead guilty after discussing the evidence with prosecutors and out of a desire to bring closure to victims, said his attorney, Marsh Halberg.

"We won't have to bring children to court to testify," Halberg said.

Martynenko, who will stay in federal custody, wore an orange prison jumpsuit as he addressed U.S. District Judge Richard Kyle on Thursday. Martynenko's mother and an uncle attended the hearing, and became emotional as the case's facts and sentencing possibilities were read.

"It's a sad, sad day for everybody," Halberg said.

Though Martynenko's case stood out for its scope, Kayser said "sextortion" is a fast-growing crime — something she hopes puts parents on alert.

"It's interesting when you talked to the kids: They really believed they were talking to a woman," Kayser said.

Stephen Montemayor • 612-673-1755