Sicko R. Kelly's Secret Sex Dungeon Exposed!

Sleazy singer R. Kelly secretly constructed a "heavily guarded" dungeon where he allegedly lured yet another underage girl to perform humiliating sex acts!

That's the shocking claim of an exhaustive RadarOnline.com investigation, which insiders said may lead to a federal probe of the twisted singer.

"R. Kelly could be in real trouble if the feds begin investigating him," said a confidant familiar with the R&B superstar's mounting problems.

"What they find could put him away for a long time!"

His latest accuser, Jerhonda Pace, charged on Aug. 22 she had sex with the "I Believe I Can Fly" singer multiple times in his massive Chicago mansion before she was 17 — the age of consent in Illinois.

Pace claimed the sick star filmed their encounters in his private sex lair without her knowledge or permission and he humiliated and physically abused her .

"He was very controlling," said the shaken woman, who said she was a virgin when she first met Kelly in 2008 as he faced child pornography charges.

After she performed oral sex on the singer, Pace claimed Kelly forced her to write a statement saying she'd stolen his jewelry and cash, and her parents were forcing her to blackmail him.

Pace is the latest woman claiming to expose Kelly's tawdry tactics amid charges he runs a brainwashed "cult."

"Kelly is a master at mind control," insisted Cheryl Mack, his former personal assistant.

In 2002, it was first reported that Kelly allegedly kept a secret sex den at his Chicago estate, where he subjected young girls to orgies, obscene fetishes, role-playing and spankings.

Kelly's attorney vehemently denied the allegations, saying they have been made by "individuals known to be dishonest … with personal agendas."