More than a year after an alleged rape at a lavish Georgetown Halloween party, prosecutors have decided not to bring charges against the man accused of the assault.

"The U.S. attorney's office for the District of Columbia and the Metropolitan Police Department thoroughly investigated this matter," said William Miller, a spokesman for the U.S. attorney's office. "Following this investigation, the U.S. attorney's office determined that there is insufficient evidence to go forward with charges at this time."

The alleged victim, now 22, told police and a grand jury that she was sexually assaulted at the $5 million home known as Dodge Mansion owned by Bill Dean, the chief executive of M.C. Dean, one of the country's biggest electrical contractors. She identified her alleged assailant as one of Dean's friends.

The Washington Post is not naming him because he was not charged with a crime. The man, a married lawyer, declined to respond to messages left at his law firm and with one of his attorneys. His accuser did not respond to messages but posted an angry denunciation of him online in late October.

"[You] are a disgusting excuse of a man," she wrote. "All I want to say is: I am glad I pressed charges. I don't represent myself but every other girl silenced and forced to carry this experience . . . in their life and didn't get a voice. That's what made me fight for one."

(The Post generally does not identify alleged victims of sexual assault.)

[Bill Dean throws wild Georgetown parties. Now a rape is alleged at one.]

The woman's parents, who live in Frederick County, Md., said in interviews that the investigation's length had convinced them that charges would never be brought. Their daughter testified before a grand jury in March. One of the prosecutors sent them a discouraging letter last month, shortly after the one-year mark of the party.

"What I told your daughter is that I truly believe her, however my legal obligation is something different," wrote Nicholas Miranda, an assistant U.S. attorney. "My legal (and ethical) obligation is to only go forward and try to indict cases where after a complete review of the evidence I believe that the case can be proven to a jury to the highest standard — beyond a reasonable doubt."

The young woman's mother said the decision not to bring charges, made earlier this month, only confirmed their worst fears.

"Our daughter felt let down by the system, we think," said the mother. "It's disappointing that the system doesn't appear to work, even with all this stuff in the news" about sexual harassment and assault. "People wonder why victims don't bring up allegations, and that's because the justice system doesn't work."

Though she spoke to police the night of the party, it took the alleged victim until January to file a complaint. Her mother drove her to the District to meet with a police detective.

"She braved it to tell her story," her mother remembered. "But she was a mess, and I thought, 'Are we setting her up for disappointment?' "

That month, their daughter, who had dreamed of becoming a teacher, dropped out of college.

In interviews with The Post last year, she recounted the alleged assault in detail. She said a friend invited her to the election-themed Halloween party, where a few hundred guests, including topless models covered in body paint, mingled with Washington wonks. She'd never been to Dodge Mansion. After she showed up, she headed to an upstairs bedroom, where she and other partygoers snorted cocaine, she said. The drug made her panic. She went to the bathroom, and met up with a group of women.

But then, she said, the alleged assailant entered and the two wound up in a connected bedroom.

"He locked the door, and he started touching me, and I realized what was going on. I said, 'Can you stop?' He wasn't stopping," the woman told The Post. He violently bit and grabbed her. Then he raped her, she said. "It was the most excruciating pain," she said.

Moments later, Stephanie Larimore, a model and former Playboy playmate, entered the room and saw that the alleged victim was being comforted by another woman.

"She was telling us that she had been attacked, raped, and she was lying on the ground naked, crying," Larimore told The Post. When Larimore and the other woman began to take the alleged victim to another room, the alleged attacker tried to stop them. He threw one woman against the wall and knocked Larimore to the floor, banging her head.

Dean did not respond to messages seeking comment.

In an interview Monday, Larimore said she testified before the grand jury in November, describing what happened to her and what she'd seen.

"You have to have witnesses to testify, and I think I was the only one who testified on the victim's behalf," she said. "If it was another guy with a criminal background, they would have locked him up right away without any questions asked. But since it's this man and the fact he hangs out with someone very powerful, I don't think they really care or wanted to deal with it. And that's really sad. He knows he can get away with whatever he wants."