TMZ has obtained FBI documents detailing shocking allegations of sexual abuse involving minors and Hollywood types ... and it's simply shocking.As TMZ previously reported ... Michael Egan is suing "X-Men" director Bryan Singer for sexually assaulting him numerous times at a party at a mansion in Encino, CA. Singer denies it. Egan claims he went to the FBI in 2000 but sources tell us he NEVER complained about Singer.It appears, however, Egan DID complain about the men who owned the mansion -- Marc Collins-Rector and Chad Shackley. TMZ has obtained an affidavit written by FBI Special Agent Joseph Brine. We know Egan -- who was 15 at the time of the alleged assault -- is referred to as Minor #4.In the affidavit -- dated May, 2003 -- Egan says Shackley's younger brother invited him to a graduation party at the Encino estate. According to the docs, there were 5 to 6 people present, including Rector and Shackley -- Singer is not mentioned. Egan claims during the party, Rector told him "90 percent of show business was gay and that you needed to sleep with people if you wanted to go anywhere." Rector allegedly warned, "We stay together, but you do not want to see my dark side," and then displayed a gun.Rector allegedly said he was one of the 25 most powerful people in Hollywood and it would be a mistake to make the group angry.What followed, according to Egan, was a barrage of sexual assaults at the estate and elsewhere. Most shockingly, Egan says he was taken to a Siegfried and Roy show in Vegas, drugged and when he awoke he felt a pain in his rectal area and realized he had been sodomized by Shackley.Collins-Rector was charged with 21 counts of sexual assault. He copped a plea to 1 count. We're told Collins-Rector and Shackley fled the country.The other minors who are mentioned in the complaint tell wild stories ... being flown from various states to California, offered $100K a year for college tuition, cars and other perks in return for sex.One minor was allegedly told by Rector if he refused to have sex he'd be sent back to his "hick town" and never have a life.