DPS is investigating a fatal 18-wheeler crash early Wednesday on Highway 6 in Grimes County that led to the death of an Amarillo man. According to DPS Sergeant Justin Ruiz, the crash is believed to have occurred just after 1:45 a.m., but was not reported until around 6:30 a.m.Ruiz said a 2014 Freightliner was headed southbound on Highway 6 when it went off the roadway to the left, struck a guardrail and crashed into Grassy Creek, just south of Navasota city limits. The driver of the 18-wheeler, identified as 33-year-old Isack Aguilera, was pronounced dead at the scene. No other occupants were in the vehicle, and no other vehicles were involved. Investigation is ongoing.