Superman actress Margot Kidder's death has been ruled a suicide, and her daughter said Wednesday it's a relief to finally have the truth out.

Kidder, who played Lois Lane opposite Christopher Reeve's Superman in her most famous role, was found by a friend in her Montana home on May 13.

At the time, Kidder's manager, Camilla Fluxman Pines, said Kidder died peacefully in her sleep.

A statement released Wednesday by Park County coroner Richard Wood said she "died as a result of a self-inflicted drug and alcohol overdose" and that no further details would be released.

Maggie McGuane, Kidder's daughter by her ex-husband Thomas McGuane, told The Associated Press in a phone interview that she knew her mother died by suicide the moment authorities took her to Kidder's home in Livingston, a small town near Yellowstone National Park.

Kidder signs autographs at Comic Con in San Diego, Calif., in July 2005. (Sandy Huffaker/Getty Images)

"It's a big relief that the truth is out there," she said. "It's important to be open and honest so there's not a cloud of shame in dealing with this."

Kidder's death is one of several high-profile suicides this year that include celebrity chef Anthony Bourdain and fashion designer Kate Spade.

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McGuane noted that Montana has one of the highest suicide rates in the nation and she urged people with mental illness to seek help.

"It's a very unique sort of grief and pain," McGuane said. "Knowing how many families in this state go through this, I wish that I could reach out to each one of them."

Kidder is seen with Prime Minister Pierre Trudeau as she arrives at the premiere showing of the film Louisiana in August 1984 in Montreal. (Moe Doiron/Canadian Press)

Kidder struggled with mental illness much of her life, and it was made worse by a 1990 car accident that left her in debt and led to her using a wheelchair for almost two years.

Kidder and Reeve starred in four Superman movies between 1978 and 1987. She also appeared in The Great Waldo Pepper with Robert Redford in 1975, Brian De Palma's Sisters in 1973 and The Amityville Horror in 1979.

She later appeared in small films and television shows until 2017, including R.L. Stine's the Haunting Hour. She received a Daytime Emmy Award as outstanding performer in a kids' series in 2015 for that role.

'Blazing energy'

Kidder, a native of Yellowknife, was also a political activist who was arrested in 2011 in a Washington, D.C., protest over the proposed Keystone XL pipeline.

Her final years were troubled by conflicts with people who were down on their luck that she took into her home. Between August 2016 and her death in May, authorities were called to her house 40 times on reports of people trespassing, theft and other disturbances, according to police logs released to the AP under a public-records request.

Kidder and other environmentalists protest outside the White House in Washington in August 2011. (Keith Lane/Canadian Press)

The calls include responses by ambulances five times in seven months, including at the time of her death.

Joan Kesich, a longtime friend who found Kidder's body, said Kidder was fearless and always spoke the truth, regardless of the consequences.

"In her last months, she was herself — same kind of love, same kind of energy," Kesich said. "The challenges that she had were very public. I want what I know about her to be out there because it was glorious. She was really a blazing energy."

Where to get help:

Canada Suicide Prevention Service: 1-833-456-4566 (Phone) | 45645 (Text) | crisisservicescanada.ca (Chat)

In Quebec (French): Association québécoise de prévention du suicide: 1-866-APPELLE (1-866-277-3553)

Kids Help Phone: 1-800-668-6868 (Phone), Live Chat counselling at www.kidshelpphone.ca

Canadian Association for Suicide Prevention: Find a 24-hour crisis centre

If you're worried someone you know may be at risk of suicide, you should talk to them about it, says the Canadian Association for Suicide Prevention. Here are some warning signs: