Fuller, 24, was found hanging from a tree in the park Wednesday.
"My friend was the sweetest person you know. He had the biggest smile, the best laugh," Tommie Anderson says. "He could make two people who didn't like each other sit in a room together."
Anderson says Fuller was her best friend.
The Los Angeles County Sheriff's Department says the cause of death appears to be suicide, but authorities are waiting for full results of an autopsy before making a final determination.
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Anderson does not believe Fuller would take his own life.
"I feel like somebody murdered my best friend and they used this tree to cover it up," Anderson says. "There's a lot of Black Lives Matter things going on and I feel like they want to put it on - it was mental issues, it was depression, it was something other than the fact that a young man was hung from a tree."
Fuller's family called for justice during a candlelight vigil Saturday night.
L.A. County Supervisor Kathryn Barger and local lawmakers want the state attorney general to investigate Fuller's death.
Community leaders are calling for a federal probe.
"So now we have a hanging in Palmdale. No conclusions, we're not judging anything. Just simply an independent investigation by the FBI," Earl Ofari Hutchinson of the Los Angeles Urban Policy Roundtable says. "Let's determine the facts, what happened to Robert Fuller."
Sheriff Alex Villanueva will hold a morning news conference Monday to address the death.
If you or someone you know needs help, do not hesitate to call the National Suicide Prevention Lifeline at (800) 273-8255.
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