LOS ANGELES -- An assistant to Britney Spears has been subpoenaed by attorneys representing the star's ex-husband, Kevin Federline.

The New York Daily News reported that Federline hired an ex-Israeli commando to serve Britney’s assistant Alli Sims with legal papers.

Aaron Cohen, described as a "security expert," was dispatched by Federline to hunt down Sims while she partied with friends Saturday night.

Federline's attorney said the subpoena was delivered but declined to say why Sims would be questioned.

Spears officially divorced Federline, a rapper and dancer, on July 30.

It was not known whether the assistant's testimony would be related to a child custody hearing scheduled for Tuesday.

Since February, the couple have shared joint custody of their two sons, Sean Preston and Jayden James.