Law-enforcement sources told the celebrity news Web site Wagner was arrested April 22 on cocaine and heroin possession charges after police were called to her home to investigate a complaint of a loud fight involving a gunshot.
No injuries were reported in the incident.
TMZ said Monday an unidentified male was arrested at the scene for negligent discharge of a firearm.
Wagner posted $10,000 bail but it was unclear if the other person is still in custody, the Web site said.
Courtney Wagner's mother, Natalie Wood, a star in "Miracle on 34th Street," "West Side Story," "Splendor in the Grass," "Gypsy" and other movies, died in November 1981. Her death was ruled an accidental drowning.
Robert Wagner, 82, starred in the TV shows "It Takes a Thief," "Switch" and "Hart to Hart."