Police in Winnipeg said Thursday they do not know what triggered a vicious attack on a Greyhound bus the night before that led to the beheading of a passenger.

Witnesses say a man was stabbed to death and then decapitated in what appears to be a random act of violence on board the bus that was en route to Winnipeg late Wednesday.

At a news conference Thursday afternoon, RCMP Staff Sgt. Steve Colwell said a 40-year-old suspect is in custody and police were preparing to interview him. No charges have been laid.

Colwell would not confirm passenger reports that a man repeatedly stabbed his seat-mate before beheading him.

He said passengers had already left the bus when officers arrived at the scene near Portage la Prairie, Man. He added that police apprehended the suspect when he tried to escape the bus by jumping out a window.

Colwell said the actions of the passengers and driver may have prevented any other attacks from occurring.

"It's not something that happens regularly on a bus," said Colwell. "You're sitting there enjoying your trip and then all of a sudden somebody gets stabbed. I imagine it would be pretty traumatic ... the way they acted was extraordinary."

"They were very brave. They reacted swiftly, calmly in exiting the bus and as a result nobody else was injured."

Shocked witnesses said the victim, described as between the ages of 18 to 20, was sleeping with his head against the window when the attack occurred.

Garnet Caton, who was sitting in front of the victim, heard the commotion and turned around thinking he was going to witness a fight.

Caton told CTV News that once he realized what was going on he screamed: "Stop the bus, somebody's getting stabbed, everyone get the hell off."

The bus driver pulled over on a section of the east-bound Trans-Canada Highway -- about 15 kilometres west of Portage la Prairie -- and many of the 37 passengers began to flee the bus.

Passengers attempt rescue

Caton gathered a small group of people to go back and help the victim, said CTV correspondent Murray Oliver in Winnipeg.

"They returned to the back of the bus to find that the person who was stabbing the person in the neck had now sawed off the head of (the victim)."

The man, with the head in one hand and the knife in the other, then tried to attack the other passengers, said Oliver.

The group was able to exit the bus and slammed the door behind them.

The man then stabbed the door with his knife but was unable to break through and get off the bus. Eventually, he attempted to start up the bus to drive away but the operator had hit a switch, disabling the vehicle.

Oliver said a truck driver then arrived at the scene and handed out wrenches and crowbars to several men. The small group gathered around the door to the bus and prevented the man from exiting until police arrived.

Cody Olmstead, a Nova Scotia man who was on the bus, said that the killer taunted the men who were blocking the door from the outside.

"He cut (the victim's) head off and then walked up to the door holding it and just looked at them crazy-like and then dropped the head and walked back to the body and started cutting it some more," Olmstead told CTV News from outside a hotel in Brandon, Man., where he and other passengers were taken.

The man was left alone with the body and witnesses say that he performed further indignities to the victim.

"We have word from people on the scene . . . that when the killer was alone with the body . . . that there may have been some small acts of cannibalism on the body," Oliver said.

Oliver said that police have not confirmed that, but have not denied it either.

Criminal profiler Pat Brown told CTV Newsnet that the killing may have been planned in advance, even if the choice of victim was completely random.

"This may have been planned -- to have this moment of rage," she said.

A number of witnesses referred to the suspect's actions as "robot-like." Brown said that fits the profile of some high-profile psychopaths who go into a "trance" while performing ghastly acts, ignoring the world around them.

The suspect

RCMP eventually arrested the man after a standoff lasting several hours, said Oliver.

Witnesses said the suspect was tall, large and wearing sunglasses, even though it was dark.

David Eastwick, a passenger on the bus, told CTV News that he noticed that there seemed to be something "wrong" with the suspect even before the attack.

"He look kind of suspicious when I saw him," a clearly-tired Eastwick said Thursday. "Most people if you say 'Hi' to them they say 'Hi' back to you but this one just looked back and (stared.)"

"I knew something was wrong (with) his mind."

Greyhound provides counselling

Abby Wambaugh, media relations spokeswoman for Greyhound, said the company is now working with Transport Canada to review bus security.

She called the situation tragic but said travelling by bus is still "the safest mode of transportation in the country."

Wambaugh also said Greyhound is fully co-operating with the RCMP investigation.

Passengers will be escorted to Winnipeg once they are interviewed by RCMP, said Wambaugh.

Once there, Greyhound will provide counselling for any passengers who want the service, she said.

Greyhound released the following statement Thursday night:

The incident near Portage la Prairie, Manitoba, was tragic. Our condolences go out to the victim's family, and Greyhound Canada will continue to cooperate fully with the RCMP on their investigation to determine why exactly this horrible event occurred. Intercity bus has been and remains the safest mode of transportation in the country.

Government responds

Meanwhile, Public Safety Minister Stockwell Day said Thursday he was shocked when he heard about the incident.

"Like most Canadians I'm horrified to hear of the account," Day said. "It's more than most people can even contemplate."

He said it was "probably one-of-a-kind in Canadian history."