Kevin Federline’s subpoenas Britney Spears’s former assistant, Shannon Funk

Federline has been issuing subpoenas to Spears’ staffers in an effort to build a case against his ex-wife in the custody battle over their children, Sean, 23 months, and Jayden, 11 months, reports US Magazine.

Last week, the singer's former bodyguard Daimon Shippen was served with papers, Spears' assistant Alli Sims was served as well.

"Kevin is pursuing relevant information from all relevant sources...There is a significant strategy in place that Kevin is going to pursue whatever he needs to do to protect his kids," says Kevin Federline’s lawyer Mark Vincent Kaplan