(CNN) -- As horrified travelers watched, a Greyhound Canada bus passenger repeatedly stabbed and then decapitated a young man who was sitting and sleeping beside him, a witness said Thursday.

Police investigate the scene near Portage la Prairie, Canada, on Thursday.

"There was a bloodcurdling scream. I was just reading my book, and all of a sudden, I heard it," Garnet Caton, who was sitting in front of the two men, said of the Wednesday night incident west of Portage la Prairie in Manitoba.

"It was like something between a dog howling and a baby crying, I guess you could say," Caton said. "I don't think it will leave me for a while."

Passengers exited the bus, and a trucker who stopped provided wrenches and crowbars to several of them so they could keep the suspect on the bus until police came, witnesses told Canadian TV.

The suspect was seized with the help of negotiators, Royal Canadian Mounted Police Sgt. Steve Colwell said.

He said no formal charges had been filed, and he declined to identify either the man in custody or the victim, who were among 34 passengers.

The was no immediate indication of what prompted the attack, Colwell said. He said he didn't know how many times the victim was stabbed. Witnesses described the weapon as a large butcher-type knife.

Caton told The Associated Press that the victim appeared to be about 19 years old and had gotten on the bus in Edmonton.

Colwell praised the "extraordinary" level-headedness and bravery of the bus driver and passengers[...].