"ER" actress Mary Mara's caused of death was confirmed on Thursday to be "an accident" and "due to asphyxiation, and water drowning" while she was swimming in a New York river over the weekend. Mara was 61. "The Jefferson County Medical Examiner’s Office performed a post-mortem examination on the decedent and determined that Mary Mara’s manner of death was an accident, due to asphyxiation, and water drowning," New York State Police Officer Troop D Public Information Officer Jack Keller told Fox News Digital.