Amanda Eller's SUV was found parked at the Makawao Forest Reserve on Maui on Thursday. The 35-year-old's cell phone and wallet were still inside, and her keys were hidden under a tire, where her friends say she often left them when she went running in the park's trails.
Eller is a regular hiker and avid outdoorswoman, friends said, but she has not been seen since taking to the trail. Friends are now handing out fliers and sharing pleas for answers on social media.
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"I had just seen her a week ago. When I saw her, she was so happy. She just was glowing," Eller's friend Lucia Maya told ABC News.
"I think it's 50% she's lost in that forest, she's injured and we need to find her, because there are a lot of steep ravines...there're cliffs," friend Sarah Haynes said. "The other 50% is, 'Did somebody harm her or take her?'"
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"God forbid something's happened to her. If anybody has Amanda, please let her go. We don't want you. We want her back," Haynes added through tears.
Eller's boyfriend told police he last saw her Wednesday morning. He is not a suspect, friends say.
As volunteers work to track Eller down, police told ABC News that the FBI and multiple other agencies are now involved in a search that, for now, has very few leads. Friends are asking for people with experience navigating rugged terrain to assist with search efforts.
There is a $10,000 reward for information leading to her safe return.
ABC News contributed to this report.