Michael Smegal allegedly sent the letter to Van Nuys Airport in December. Prosecutors also say he'd admitted sending letters and packages to Foster in 2004 and had promised to stop.
Federal prosecutors say the letter was identical to more than 100 others that were sent to celebrities, businesses, airports and other institutions in the Los Angeles area between last September and January this year.
An FBI agent says Smegal chose airports because he thought they had a connection with the Screen Actors Guild, which Smegal believed had made disparaging remarks against Foster.