Sources tell both publications that the August incident outraged Halderman, and is what led to his alleged attempt to extort money from the talk show host.
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."
If this is indeed true, it is contrary to earlier reports that Letterman ended the relationship before the birth of his son in 2003. And according to the Post:
It also appears to explain why Halderman waited until September to hatch the twisted revenge plot -- rather than set it off last December when he first discovered the affair through Birkitt's salacious diary entries and e-mails to Letterman.
In March, Letterman married Regina Lasko, his girlfriend of 23 years and the mother of his 6-year-old son. Letterman has said on the air that he is trying to repair his marriage.
A Letterman spokesman had no immediate comment about the latest reports.
Halderman is free on bail, after being charged with demanding $2 million from the talk show host to stop him from writing a screenplay about Letterman's behavior.