BEIJING, Aug. 20 (Xinhuanet) -- Kevin Federline took another step in the public battle with ex-wife Britney Spears for child custody when his defense team delivered a subpoena to the Promises rehab center where Spears spent a month undergoing treatment.

Aspiring rapper Kevin Federline watches an NBA basketball game between the Atlanta Hawks and Miami Heat in Atlanta, Georgia Nov. 18, 2006. (Xinhua/Reuters, File Photo)

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Federline's attorney, Mark Vincent Kaplan, confirmed to "Access Hollywood" an "administrative executive" at the facility was served with the subpoena on Saturday, as Kevin and his camp continue to build their case in over the couple's two children ─ Sean Preston and Jayden James.

Promises is just the latest target of Kevin's camp. Earlier this week, Britney's former assistant Shannon Funk, former bodyguard Daimon Shippen and friend and current assistant Alli Sims were all served with legal papers.

"They are pursing all relevant evidence from every potential source and every available witness, including witnesses who might actually believe they could succeed in becoming unavailable," a source told "Access Hollywood."

(Agencies)