Medical examiner spokeswoman Ellen Borakove told E! News Richardson's death was ruled an accident Thursday following an autopsy.
The 45-year-old, British-born actress died Wednesday after she was taken off of life support, following a skiing mishap on a bunny slope in Canada that resulted in an injury that reportedly left her brain dead.
E! News said a private memorial is being planned by the Greenwich Village Funeral Home in New York.
Ralph Fiennes, who co-starred with Richardson in "The White Countess" and "Maid in Manhattan," and who acted opposite her husband Liam Neeson in "Schindler's List," told People.com: "For everyone who knew and loved her, Natasha's death is a terrible, devastating loss. She was a star. A great actress, a beautiful woman, a fiercely loyal friend, a brilliant and generous companion. She was an adoring and loving wife and mother. She was unique. My thoughts and prayers go out to Liam and her beautiful sons, Michael and Danny and to all her family. I cannot imagine a world without her wit, her love, her mischief, her great, great talent and her gift for living. I loved her very much. She was a supreme friend. I shall miss her deeply."
Richardson's "Cabaret" co-star Alan Cumming described her as a "life force" in "Tasha," an essay he wrote on his blog.
"I have been thinking about the times I spent with her since I heard the news of her tragic accident and the strongest memory I have is of her laughter, her unmistakable throaty laugh. I think that's a great way to remember someone," Cumming said.