Joe Tacopina, Myla Sinanaj's attorney, tells TMZ ... Kris has gone on a campaign of destruction against her to save his own [rear] in his divorce from Kim Kardashian. TMZ broke the story, Kris' lawyers contacted the FBI, claiming Myla tried to extort him by demanding a huge sum of money or else she'd spill secrets about their relationship to the media.

Tacopina tells us, "I defy them to show that she has asked for one red cent," adding, "I challenge them to submit Kris Humphries to a polygraph and if he does I will have my client do the same. She never asked him for a penny."

But there's more ... Tacopina says Kris is a boldfaced liar by claiming Myla was a booty call. Tacopina says, "I have hundreds of text messages and phone records which show he constantly contacted her on a steady basis. He'd call her 50 times in a day."

Tacopina says Kris told Myla when pictures of them first surfaced that they had to hide the fact that they were in a relationship. Tacopina says Kris didn't want to screw up his legal war with Kim Kardashian, adding, "It's hard to argue you're heartbroken when you had a steady girlfriend since January."