Montana lawyer Roger Renville said he was surfing the Internet on Saturday when he found the 10-minute video on an Alberta-based website.
Renville's find is now believed to be at the centre of a bizzare case in Montreal where a man was killed and parts of his body were mailed to political offices in Ottawa.
Renville says he tried to bury the video in his mind but couldn't, so he decided to go to police on Sunday.
He said he called his local sheriff's department, the FBI in Denver and police in Miami, but was rebuffed at each turn. He went back and looked at the video and trolled the site for clues. A poster in the background matched another video that pointed to a Canadian man, so Renville called police in Toronto.
But he says the official on the other end wasn't interested.
"He told me that what I was seeing was almost certainly fake — that special effects were very good," Renville recalled Thursday.
"I told him, 'You have to see the video to determine that.' He told me my story didn't make sense and 'why would a killer film his own crime and then post it on the Internet?'
"I asked him to at least give me an email address and I would at least send him the link to the video and he said 'no.'"
The video shows a naked man tied to a bed in a dimly lit room with a poster of the movie "Casablanca" on the wall behind it. Another man wearing a dark hoodie begins stabbing the individual and a series of edits show the body eventually dismembered and mutilated.
Renville said it was obviously real.
"The first question with any of those videos is real or fake? But with that one it doesn't take long to decide real. From the first second you see two living and breathing human beings, and within the first couple of minutes there's only one living and breathing human being left."
Toronto police confirmed that they received a call from Renville late Sunday evening and that the operator suggested Renville call Crime Stoppers.
Const. Wendy Drummond said the response was reasonable given the information the call-taker had.
"Obviously hindsight is 20-20 and would we do different if we had the information? We can only act with the information we had at the time."
On Tuesday, a severed foot was sent in the mail to the Conservative party office in Ottawa. A severed hand was intercepted before it arrived at Liberal party headquarters.
A suitcase containing a human torso was found in Montreal and, on Wednesday, police there named Luka Rocco Magnotta as the prime suspect. He is now the subject of a worldwide manhunt.
Montreal police confirm they are in possession of a video that apparently shows the slaying but have not commented further on the contents.
A spokesman confirmed Thursday that police are trying to have the video on the Internet removed but everytime it goes down in one place it crops up somewhere else.
"We discovered the whole video yesterday (Wednesday) and we've done everything to remove it, but people are putting it back (on the web) and this is hard to believe," said Cmdr. Ian Lafreniere, a Montreal police spokesman.
"It's (weird) that we have to ask people not to put a video out of someone kiling another human."
The website, reportedly run by Mark Marek of Edmonton, still had the video up as of Thursday. Marek has said he originally thought it was a bad joke and could have been made by someone with access to dead bodies.
French-language paper La Presse quoted Marek as saying he wouldn't take the video down because to do so would be like hiding your head in the sand about a real event.
Edmonton police say they have not received any complaints about the video and are not involved in the investigation involving Magnotta.
"It is not illegal for these websites to share this video so apparently law enforcement agencies can't force anyone to take it down unless of course the individual was involved in the crime," said Patricia Thenu from the Edmonton Police Service.
Some viewers posted their approval on the website.
"It's about time more (visuals) like this came up, been looking forward to seeing this one, love this," said one message.
"It was okay, the sound was the only nasty effect, I would of liked to see something worse," read another.
Not everyone was giving it a thumbs up.
"I’ve been a regular of this site and this is the first time I was like shocked by what I saw. Yes, if anyone knows at least the victim please call authorities. This dude has serious issues and well ... Please do anything!"