Discuss as:

Parents' Facebook sting helps catch sex offender

SEDRO-WOOLEY, Wash. - A 19-year-old registered sex offender has been arrested in Skagit County, Wash. after the mother of the girl he was dating set up a Facebook sting.

Jesper and Julie Myrfors say they had been concerned about their daughter’s boyfriend.

"We just thought she was getting into a dangerous situation,” said Jesper.

"When I met him, he was totally charming, really likable,” said Julie.

He is 19-year-old William Elms, also known as Liam. Liam was dating the Myrfors' 17-year-old-daughter. Then, the couple learned Liam was a registered sex offender.

Read the full story at KING 5

"When there's a question, follow it.  Find the answer for your kids,” said Julie.

With some help, the parents set up a sting. The Skagit Valley Herald reported that the couple created a fake profile of a 15-year-old girl, linked to a mutual friend of their daughter.

After a short time, Elms began sending explicit messages and pictures to the ‘girl’.

"My daughter didn't know we had done this. I invited her on to begin watching live.  She started watching conversations on our fake little person,” said Julie. "The hardest part as a parent was watching her heart break."

The relationship was over, but the story wasn't.

"If we were just to say, well, our daughter's off the hook.  We would be bad citizens,” said Jesper.

After the couple informed police, Elms was arrested at the Snohomish County Jail where he was serving time for a probation violation. He's now back in the Skagit County Jail, facing charges of Immoral Conduct with a Minor.

The Associated Press reported Elms’ arrest took place on August 18.

KING 5 is an NBC affiliate based in Seattle.

More content from NBCNews.com:

Follow US News from NBCNews.com on Twitter and Facebook