Halderman lived until recently with Birkitt, who had previously been a personal assistant to Letterman. Public documents indicate that Birkitt and Halderman lived together in Norwalk, Conn., from August 2005 to August 2009. Birkitt now has an apartment on Manhattan's Upper West Side.
Halderman has been married and divorced twice, according to public records. He had two children with his second wife, and has three stepchildren. His former wife won custody of their children and now lives in Colorado.
Divorce records show that Halderman was ordered to pay $6,800 a month in alimony and child support, starting May 1, 2004.
Halderman has been suspended from his job as the investigation continues.
ABCNews' Emily Friedman and Alice Gomstyn contributed to this report.