Paul Walker Family Blindsided By Sheriff's Death Report

3/26/2014 9:30 AM PDT

Paul Walker
's family did not get an advance courtesy call from law enforcement telling them they had determined the cause of Paul Walker's death ... and they're not happy.

Walker family sources tell TMZ, they feel totally betrayed by the L.A. County Sheriff's Dept. ... the agency that conducted the probe.

The Sheriff's Dept. held a news conference Tuesday saying speed caused the crash -- not mechanical failure.  The family found out through media reports.

The family says they've been waiting for months, and it's been very difficult preparing for the moment when they learned why Paul died.  They say after such a long, emotional wait, they now feel shortchanged.

One family member learned of the results Tuesday on TV ... when the news ticker at the bottom of the screen unexpectedly rolled through.  The family member says that was simply cold.

We reached out to the L.A. County Sheriff's Dept. for comment -- so far, no word back.