California Highway Patrol press release: 

On 06/27/2017, at approximately 0945 hours, a 1993 Motor home driven by Harlan Myers of Portland, OR was traveling north on US-101, approximately one tenth of a mile south of Dora Creek approaching active construction zone.  Due to the construction northbound US-101 traffic was stopped and backed up just south of Dora Creek. An unmarked patrol truck (2013 Chevrolet) assigned to an investigative services unit being driven by Deputy Smith of the Mendocino County Sheriff Office was stopped in the northbound lane at the end of the backed up traffic. The Chevrolet was behind a 2007 Honda CRV and a 2004 GMC Yukon.  For reasons still under investigation, Myers failed to stop the motor home and collided with the rear of the Chevrolet.  This set off a chain reaction causing the front of the Chevrolet to collide with the rear of the Honda and then the front of the Honda to collide with the rear of the GMC.  As a result of the collision, the passenger of the Chevrolet (Trouette), a citizen on a ride-along with Deputy Smith, sustained moderate injuries and was transported to Howard Memorial Hospital by City Ambulance of Eureka.  The front passenger of the GMC (Mora) sustained moderate injuries and was transported to Jerold Phelps Community Hospital by City Ambulance of Eureka.

Cal Fire and City Ambulance assisted with medical aid.  The northbound lane of US-101 was shut down for approximately 30 minutes and Cal Fire assisted with traffic control during the incident.

This incident remains under investigation by the Garberville CHP Office and alcohol or drugs does not appear to be a factor.