David Letterman and Stephanie Birkitt
John P. Filo/CBS
10/08/2009 AT 12:00 PM EDT
It wasn't the $2 million he allegedly sought that prompted Joe Halderman to try to blackmail David Letterman
– it was jealousy, according to a source.
In August, Halderman, 51, caught the talk-show host and his then-girlfriend, Letterman's onetime assistant Stephanie Birkitt, passionately kissing in a car in the driveway of the Norwalk, Conn., home Halderman shared with her, a source tells PEOPLE.
Letterman and Birkitt "were seen making out in the driveway in August," the source says. "He drove her home once a week. The interesting thing was he lives in North Salem, N.Y. Driving her home to Norwalk wasn't exactly the next town."
The PEOPLE source could not confirm a report in the New York Post
that Birkitt agreed to end her relationship with Letterman after being caught with him by her boyfriend.
But the source did say that, shortly after she was discovered with Letterman earlier this year, Birkitt and Halderman parted ways, and Birkitt moved to Manhattan.
Previous reports had suggested that Letterman, 62, had broken off his relationship with Birkitt, 34, before the birth of his son in 2003. He married his longtime girlfriend, Regina Lasko, in March 2009, and publicly apologized
to her on the air after breaking the news of the alleged extortion scheme.
The source would not confirm how long Letterman and Birkitt remained together romantically.
Reporting by LIZ MCNEIL