Margot Kidder's death has been ruled a suicide died, her daughter has revealed.
In a statement released on Wednesday, Park County Coroner Richard Wood said Kidder died "as a result of a self-inflicted drug and alcohol overdose."
Her body was discovered on May 13 at her home in Livingston, Montana.
Kidder's daughter, Maggie McGuane, told the AP she knew at the time that her mother's death was a suicide and it was a relief to have the truth out.
Kidder was best known for playing Lois Lane alongside Christopher Reeve in the "Superman" films. In addition to playing Lane in four "Superman" films, Kidder also appeared in a 2002 production of "The Vagina Monologues."
She was diagnosed with bipolar disorder and famously had a manic episode in 1996 and disappeared for four days.
Kidder became a U.S. citizen in 2005 (she was born in Canada) and became a very outspoken political activist.