CelebSpin! Celebrity News and Hollywood Gossip.

While Kevin Federline parties in Vegas, his defense team has struck again, delivering a subpoena to the rehabilitation facility attended by his ex Britney Spears.According to a source familiar with the proceedings, Promises Treatment Center was served with a subpoena Saturday morning. This is the fourth subpoena doled out by the Federline team in the singer’s increasingly high-profile custody battle. (Earlier in the week, Spears’s former assistant Shannon Funk, bodyguard Daimon Shippen and pal Alli Sims were all served with legal papers.)

“Kevin and his attorneys Mark Vincent Kaplan and James Simon are on a mission,” says the source. “They are pursuing all relevant evidence from every potential source and every available witness.”

A spokesperson from Promises refused to comment, saying: “This is a confidential facility and we cannot comment.”

The pop star spent a month at Promises and was released on March 21. Her manager at the time said she was “successfully” released.

According to Pepperdine University Law Professor Steven M. Cron, the Federline camp is clearly seeing how Spears took to the Promises stint.

“Strategically I don’t think they are concerned with the treatment, but how she responded to that treatment,” says Cron. “Specifically, if she participated as she was supposed to. Did she have a positive attitude, did she complete it successfully. Or was she a screw up?”