A Tulsa man is headed to federal prison for life after grooming and having a sexual relationship with a girl, starting when she was 15 years old.

Alexander Sweet took the young girl across state lines to marry her and was going to leave the country with her, prosecutors said.

The victim told the judge during the sentencing hearing that this has affected her life so much, she's scared to even go outside.

Prosecutors said Sweet is an opportunistic predator who thinks the law is wrong and he is right.

Sweet initially contacted the victim online when she was 15, after she posted on social media she had lost a family member, prosecutors said.

Sweet groomed the girl for months, then started having a sexual relationship with her, prosecutors said.

Shena Burgess was appointed to be the girl's guardian ad litem, and has been by the victim's side every step of the way.

"This is at least the end for her. That way she can move on with her life,” said Burgess.

Burgess said Sweet had brainwashed the girl and had complete control over her.

She said Sweet kept the girl isolated, turned her against everyone she knew, and made her think he was the only person who could protect her. He even took the victim to Arkansas to marry her.

"It was very difficult for her to go to court and have to look at him, then testify, against him. But I think that gave her strength because she realized he was no longer in control of her,” said Burgess.

Prosecutors said Sweet has no remorse, and Sweet told the judge he still believes what he did was right. Sweet even asked the girl to get back together while she was testifying during the trial.

Burgess said she hopes this story gives other victims strength to stand up.

"I don't know if his conviction will have an effect on anyone out there who is perpetrating on a minor. But at least if individuals know that her voice was heard by the court system, everyone had a fair day in court, both the prosecution and defense, and luckily in this case the right thing happened,” said Burgess.

The judge said one of reasons he sentenced Sweet to the max is because in the past, Sweet has bragged about being sexually attracted to minors.