Stephanie Birkitt works on the "Late Show" and lived in Norwalk, Conn., with Robert Halderman until August. The official says Halderman copied Birkitt's diary, which contained details of her affair with Letterman, and tried to use it as blackmail fodder.
The official spoke on the condition of anonymity because the investigation is ongoing.
Birkitt was an assistant to Letterman and frequently appeared on camera with the host in comedy bits.
Halderman is a producer for the true-crime show "48 Hours Mystery." He pleaded not guilty last week to extortion charges.