The producers of hit TV drama CSI: Crime Scene Investigation are stepping up security on the show's California set after cast members received a barrage of death threat mail and calls.

Sickos claiming to be fans have been getting too close to stars like Marg Helgenberger and Jorja Fox - uncovering personal phone numbers and addresses, and demanding to meet them.

A set insider says, "Producers didn't take the threats lightly. They immediately brought in a private security firm to work on the set and then warned everyone associated with the show about the dangers.

"The show is a huge hit, but it has also caught the attention of some real sickos who are obsessed with autopsies."