Britney Spears has allegedly hired a private eye to dig up custody battle dirt on ex-Kevin Federline. Britney has hired Lisa Javoric Private Investigations to find examples of Kevin’s own bad behavior that she can use against him in court.

The private eye was watching constantly Kevin as he recently partied in Las Vegas.

“The detective was onto his every move,” an insider told In Touch Magazine.

“He watched him intently as Kevin partied in public areas of the hotel. He made a note of everyone who went into his suite.”

“The buddies also propositioned a girl for a threesome, according to the witness, but when she declined, Kevin’s friend called other girls to hang out with.”

Britney Spears private eye later visited the girls to try to dig up dirt.

“He said he was working with Britney’s attorney to compile evidence for her to use in her custody battle,” the source said.

“The girl feels sorry for any mother trying to keep hold of her children, so she ha no qualms about answering his questions.

“He spent more than two hours talking to her and recorded the conversation to use as evidence.”

“It’s going to get really nasty,” one Britney friend admitted.