A family spokesperson has confirmed that a body found in a Seattle ravine belongs to Misty Upham.
Police Commander Steve Stocker said there was presently no evidence or information "to believe there is foul play".
Film-maker and Upham’s long-term friend, Tracey Rector, said that the family do not think that the actress took her own life, as previously suggested.
Her father had said that Upham had stopped taking medication for bipolar disorder and anxiety, prompting speculation of a possible suicide.
"The family has stated that, after seeing the body, they still do not feel that Misty Upham committed suicide," Rector told The Hollywood Reporter.
The actress - who had appeared in August: Osage Country and Django Unchained – had been missing for 10 days before her body was found by her friends and family.
Her father confirmed her identity.
Rector said that Upham’s family were dissatisfied by the lack of concern and assistance allegedly received by the Auburn Police Department.
"First and foremost, the family wants everyone to know that the Auburn police did not help with this situation at all," said Rector.
"They refused to help. When she disappeared on Oct. 5, the family knew something was seriously wrong — it was out of character for her to be gone so long without being in touch — and they repeatedly went to the police, who insisted there was no cause for concern."
Actress Juliette Lewis tweeted that Upham, 32, has "spoke out a lot against injustices within Native community. And had known enemies".
#RIPMistyUpham I am in shock and greif. I pray the police do a murder investigation. They're saying not "foul play" when ofcourse it is.; Juliette Lewis (@JulietteLewis) October 17, 2014
#RIPMistyUpham Misty spoke out alot against injustices within Native community. And had known enemies. Police must do an investigation.; Juliette Lewis (@JulietteLewis) October 17, 2014
I cant sleep. Too much darkness. I feel so sad about Misty. I know she has more to say. And about how she died. #RIPMistyUpham; Juliette Lewis (@JulietteLewis) October 17, 2014
Misty told me about horrible violence she suffered on NA reservations She felt she coul be murdered! Bcuz she was vocal #ThisIsNOTaSUICIDE; Juliette Lewis (@JulietteLewis) October 17, 2014
Former colleagues Meryl Streep and Melissa Leo both paid tribute to the actress.
"So so sad to hear this news — all our thoughts are with her family and with her beautiful spirit," said Streep, while Leo described the death as "such a loss... so sad, so so sad.
"I hope to that her talent is remembered more than any troubles we surely will never understand. She was a smart, delightful actor, a spirit wild and hungry. I know I and others are honored to have had Misty in our lives."