A 40-year-old man is in custody in Manitoba after a young man was stabbed and, witnesses said, decapitated aboard a Greyhound bus travelling through the province overnight.

Police officers spent the night examining a Greyhound bus where a passenger was reportedly stabbed and decapitated late Wednesday. (John Woods/Canadian Press)The RCMP would not confirm the reports of beheading, saying only that a stabbing took place around 8:30 p.m. CT on an eastbound Greyhound bus on the Trans-Canada Highway about 20 kilometres west of Portage la Prairie.

The suspect, believed to be from outside Manitoba, was arrested early Thursday morning after a standoff lasting several hours and remains in RCMP custody .

Charges have not yet been laid, and the suspect has not yet been interviewed, said RCMP spokesman Staff Sgt. Steve Colwell, adding that he could release no further information on the investigation.

The RCMP declined to identify either the suspect or the victim.

Thirty-seven passengers and a driver were aboard the bus en route to Winnipeg from Edmonton.

Passenger Cody Olmstead, 21, told CBC News he had smoked a cigarette earlier in the trip with the victim, whom he described as a man in his late teens or early 20s. The victim got on the bus in Edmonton, he said.

"I never took the time to know him, but he seemed to be OK, right, just a kid," said Olmstead, a Nova Scotia man who had been taking the bus from Alberta to Montreal.

"He just said he was going to Winnipeg going home, that's where he was from."

'*lo**-curdling scream'

Garnet Caton, who was sitting in the seat in front of the victim, said he saw the attacker stab his seatmate, a young man sleeping with his headphones on.

Caton said he heard a "*lo**-curdling scream" and turned around to see the attacker holding a large "Rambo" hunting knife above the victim, "continually stabbing him in the chest area."

"He must have stabbed him 50 times or 60 times," said Caton.

"Like, just everywhere, arms, legs, neck, chest, guts, wherever he could swing it, he got it," said Olmstead.

"It looked kind of like a scuffle or an argument, you know, and then somebody's, like, 'Knife! Knife! Run!' so I was running up the alleyway, slapping people telling them to get going, move, get off the bus. I got pushed over, some lady got pushed over, I was just making sure everybody was OK, and we all got off the bus," said Olmstead

As panicked passengers fled the bus, "the attacker was over top of the victim continually cutting him. I think the victim was gone at that point," Caton said.

Trio tried to check on victim

Caton, the driver and a trucker who had stopped at the scene later boarded the vehicle to see if the victim was still alive.

Garnet Caton said he heard a '*lo**-curdling scream' and turned around to see a man repeatedly stabbing the passenger sitting next to him. (CBC)"When we came back on the bus, it was visible at the end of the bus he was cutting the guy's head off and pretty much gutting him up," said Caton.

The attacker ran at them, Caton said, and they ran out of the bus, holding the door shut as he tried to slash at the trio.

When the attacker tried to drive the bus away, the driver disabled the vehicle, Caton said.

"While we were watching the door, he calmly walks up to the front with the head in his hand and the knife and just calmly stares at us and drops the head right in front of us," said Caton.

"They did an awesome thing, holding him in there, because if not, what would have happened?" said Olmstead.

RCMP crisis teams, including negotiators, communicated with the suspect for several hours while he was on the bus. Around 1:30 a.m., he attempted to jump from a bus window and was subdued and arrested, RCMP said.

Acted 'like he was a robot'

Caton described the attacker as surprisingly calm. "It was like he was at the beach or something. There was no rage in him. He wasn't swearing or cursing or anything. It was just like he was a robot or something."

Police cruisers arrived about 10 minutes after the attack began, he estimates, and officers began directing passengers to school buses to take them to a hotel in Brandon.

"While we were waiting on the side of the road, [the attacker] was taunting the police with the head in his hand," said Caton.

Caton described the attacker as appearing "totally normal" earlier in the journey, even chatting with a young woman as he smoked a cigarette during a break.

But when he got back on the bus, he moved his belongings from the front to a seat beside the victim in the back and about 20 minutes later began attacking the man, said Caton. "He didn't say anything to the victim at all," said Caton.