A passenger traveling on a bus across Canada's vast Western plains stabbed, gutted and decapitated a man seated next to him in an unexplained attack, a witness told media Thursday. The victim had been sleeping before he was repeatedly stabbed in the chest by a man with a large knife, witness Garnet Caton told public broadcaster CBC. The other 35 passengers and driver were jolted by "blood-curdling screams" and fled. "He must have stabbed him 50 times or 60 times," said Caton. When Caton and two others returned to check on the victim, he said they saw the attacker "cutting the guy's head off and gutting him." "While we were watching ... he calmly walked up to the front (of the bus) with the head in his hand and the knife and just calmly stared at us and dropped the head right in front of us." Police then surrounded the bus and arrested the man, he said. The Royal Canadian Mounted Police said they were investigating a "major incident" that occurred at 9 pm Wednesday (0200 GMT Thursday) on a Greyhound bus traveling eastbound from Edmonton to Winnipeg, but offered no details.