Heidi Montag's Dad Arrested for Child Sexual Abuse and Incest

Heidi Montag's father has been arrested for allegedly committing years of sexual abuse and incest with a 13-year-old girl and according to authorities it went on for years ... but sources connected to Heidi and Spencer tell TMZ ... the alleged victim has significant credibility problems.

Bill Montag was picked up by Aspen cops Thursday night. Authorities say the sexual abuse continued until the girl turned 17. The complaint says he abused the child at least 50 times.

As for the time frame ... the alleged abuse occurred between 1993 and 1998. It does not line up with Heidi or her sister Holly.

According to the docs, Bill Montag "groomed her with back rubs in her bedroom in the basement of their home," and escalated by attempting to insert a vibrator into her vagina. At age 16 he began performing oral sex. It then gets extremely graphic.

The documents describe the last incident in 1998, when he allegedly crawled into bed with her, tried to pull down her underwear and said, "I want to have sex with you." Authorities say she rebuffed him by saying, "Get the hell out of my room, no!" And he responded, "I thought that's what you wanted. I thought you loved me the way I love you."

Our sources who are very close to Heidi and Spencer tell us ... they are very familiar with the accuser who "is determined to destroy the family." They say she has stalked Heidi and Spencer at various public appearances to the point security had to remove her. Heidi and Spence believe Bill Montag will be exonerated.