DES MOINES, Iowa (AP) — The National Transportation Safety Board says crew fatigue is the probable cause of a deadly train collision in southwest Iowa. A Burlington Northern Santa Fe coal train slammed into a standing BNSF train near Red Oak on April 17, 2011. The coal trains’ conductor Patricia “Trish” Hyatt and Engineer Tom Anderson, both of Creston, died in the crash. The NTSB says Tuesday that both crewmembers had fallen asleep.

Officials say Hyatt and Anderson also failed to comply with a signal requiring them to operate at a restricted speed and stop short of the standing train. NTSB Chairwoman Deborah Hersman says the investigation draws attention to the dangers of human fatigue. The board says other factors contributed to the accident, including the absence of a system that identifies the rear of a train and stops the approaching train if a safe breaking profile is exceeded.