"She did not commit suicide," Charles said in a statement on Facebook. "We believe she ran into the wooded area behind her apartment to hide from the police. The area in question has a hidden drop off and evidence suggests that she slipped and fell off of the steep embankment when she tried to get out of a view from the road. She simply did not see the drop-off."
Upham's body was discovered on Thursday at the bottom of a 150-ft. embankment in Auburn, Washington, after she had been missing since Oct. 6. The August: Osage County and Django Unchained actress was 32.
In the statement, Charles said that his daughter had suffered from mental illness and accused the local police department of treating her poorly in the past.
"Misty was afraid of the Auburn PD officers with good reason," he said. "In an incident prior to her disappearance, the Auburn PD came to pick up Misty on an involuntary transport to the ER. She was cuffed and placed in a police car. Some of the officers began to taunt and tease her while she was in the car. Because it was dark they couldn't see that we, her family, were outside our apartment just across the street witnessing this behavior.
"They were tapping on the window making faces at her. Misty was crying and she told them, 'You can't treat me like this. I'm a movie actress and I will use my connections to expose you.' Then another officer walked up to her asked, 'Are you a movie star? Then why don't [you] complain to George Clooney!'"
On the day she disappeared, police responded to her apartment after a call that Upham may have been suicidal, but when officers arrived Upham had already left.
Auburn Police commander Steve Stocker declined to comment when contacted by PEOPLE about Charles's accusations, other than to dispute Charles's claim that Stocker himself had animosity against Misty due to a previous encounter: "I had never met Misty or talked to her," Stocker says.
Earlier, Stocker responded to the family's claims that Misty's disappearance was insufficiently investigated by local authorities, who declined to rule her an "endangered" missing person.
"Our detective was doing everything they could based on the information we had and the tips that we were getting," Stocker told PEOPLE. "We feel that we've done everything we could based on these circumstances, and we're just really sorry that this was the ending."
King's County Medical Examiner's Office determined the date Misty died was Oct. 5 but says the "cause and manner of [her] death are pending investigation."
In his Facebook statement, Charles wrote that his daughter, who grew up on Montana's Blackfeet reservation and in Seattle, was in an extremely vulnerable state.
"Imagine a 32-year-old woman with mental illness, without her medication, imagine she left in an unstable mental state," he said. "Imagine for the first time in 32 years she lost contact with everyone for 11 days."
Her devastated family and friends are heartbroken by the tragic turn of events.
"Misty loved life, she had ambition, vision and a desire to make a difference in the world she lived in," Charles said, thanking the Muckleshoot Tribe and other Tribal volunteers for their support.
A memorial fund has been set up in Upham's honor.
Reporting by GABRIELLE OLYA