Stephanie Birkitt, 34, Secret Diary at Center of Letterman Sex Extortion,

NEW YORK The pages of Stephanie Birkitt, 34, personal diary were presumably not meant for the eyes of her former lover, David Letterman, or a recent one, long-time CBS News producer Robert “Joe” Halderman.

But it was pages copied from the diary, that were the centerpiece of Robert “Joe” Halderman’s plan to extort $2 million from the late night king, threatening to expose his office sexcapades if he did not comply, according to law enforcement officials who spoke on the condition of anonymity because the investigation is ongoing.

Stephanie Birkitt, 34, was an assistant to Letterman on the “Late Show” and frequently appeared on camera with the host in comedy bits. According to her diary, Stephanie Birkitt was also once his lover, say law enforcement officials.

Later, Stephanie Birkitt became romantically involved with Robert “Joe” Halderman. The couple lived together in Robert “Joe” Halderman’s Norwalk, Conn. home until August, when they reportedly split.

Stephanie Birkitt had once worked on “48 Hours Mystery,” the show where Halderman was a well respected producer. CBS has since suspended him.

Calls to her home and her parents’ home in New Hampshire went unanswered; a message left at her father’s dental office wasn’t returned Monday.

Stephanie Birkitt moved in August to Manhattan, to a second-floor apartment of an Upper West Side co-op building. Neighbors said they did not know her well, and never saw Letterman in the building.

“She seems like an enigma,” said Iris Rutkoski. “Nobody seems to know her in the building.”

Rutkoski said she might have said hello to Birkitt in passing, but “I wouldn’t recognize her to fall over her. She was not the kind to chat with her neighbors.”

Robert “Joe” Halderman apparently copied the pages from Stephanie Birkitt’s diary before they broke up, using them together with e-mails and other information in a package left in David Letterman’s limousine that was intended to extort money from him, officials said.

Robert “Joe” Halderman’s attorney, Gerald Shargel, said Monday the charge against his client is “so obviously out of character to the point of not making any sense.” Shargel said that Letterman manipulates audiences for a living and that to think last Thursday he “gave the entire story and there’s nothing more to be said is simply wrong.”

Robert “Joe” Halderman pleaded not guilty in a Manhattan court last week to a count of attempted first-degree grand larceny.