When Margot Kidder died in May, she was remembered as the actress who brought the fictional reporter Lois Lane to life in a series of blockbuster Superman movies in the 1970s and ’80s.

Her obituary in The New York Times did not specify a cause of death. But on Wednesday the Park County coroner’s office in Montana, where she lived, revealed that the cause was suicide. A statement provided by the coroner to The Associated Press said that Ms. Kidder “died as a result of a self-inflicted drug and alcohol overdose.”

Ms. Kidder’s daughter, Maggie McGuane, said she knew that her mother had taken her own life.

“It’s a big relief that the truth is out there,” she told The A.P. “It’s important to be open and honest so there’s not a cloud of shame in dealing with this.”

In the month after Ms. Kidder died at age 69 on May 13 in Livingston, Mont., the designer Kate Spade and the chef Anthony Bourdain both took their own lives, prompting widespread conversations about how to prevent what has become a national public health crisis.