Another horseback rider called 911 to report the incident around noon, Snohomish County sheriff's spokeswoman Shari Ireton said.
The horseback rider said she and her friend were riding on the trail near the 35000 block of Mann Road when two men on dirt bikes approached them. An argument broke out, Ireton said.
"One of the dirt-bike riders revved his engine, which spooked her friend's horse, which took off running away from her," she said.
A witness told police a dirt-bike rider may have followed the woman and revved his engine again, causing the horse to rear, and the woman to fall.
She was knocked unconscious and remained hospitalized on Tuesday. Deputies were waiting to talk with the woman, Ireton said.
The dirt-bike riders were wearing helmets and didn't have license plates, so it may be difficult to identify them, Ireton said.
"We don't believe there was physical contact between the two parties at this point," she said. "We'll know more when we get the witness statement from the injured party."
Police say it could be a potential case of reckless endangerment, a misdemeanor. It wasn't immediately clear whether dirt bikes are allowed on that particular trail. It's not believed to be county property, officials said. Motorized vehicles are not allowed on county trails.
Anyone with information can call 425-388-3845.
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