Brian Miller, 27, was injured in the collision, which happened at 6:15 p.m. Thursday, according to a report from the California Highway Patrol's Clear Lake office.
The CHP report said Miller was riding a 2002 Yamaha Grizzly four-wheel ATV on private property when the collision occurred.
Miller was backing down a steep incline when the ATV overturned. The CHP report said Miller was thrown from the quad, which then landed on top of him.
The CHP said Miller – who was not wearing a helmet – sustained major injuries, including an open skull fracture and the loss of his left eye.
Alcohol and drugs are suspected to be factors in the crash, which is being investigated by CHP Officer Steve Curtis, according to the report.
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