Oregon State Police are continuing to investigate the cause of Friday morning’s fatal commercial truck crash that occurred along Interstate 84 in Hood River.   Friday at about 5:12 a.m., a 2004 Freightliner truck pulling an empty container trailer driven by 64-year-old Melvin Roy Schwark of Lyle was traveling westbound on Interstate 84 near milepost 63.   For an unconfirmed reason, the truck and trailer gradually drifted to the left and struck the guardrail on the south side of the traffic lanes.  The truck and trailer continued westbound into the center median before coming to an uncontrolled rest after colliding with a tree.   A tree section came to rest partially blocking the left eastbound lane.   Schwark was pronounced dead at the scene.  Investigators are looking into the possibility that he suffered a medical emergency while driving that may have been a factor in the crash occurring.  He was driving for J&L Transportation out of Wasco, Oregon.