Canadian police have raised fears that a serial killer may be responsible for the deaths of an Australian tourist and his American girlfriend, whose bodies were discovered in north-western Canada.

Lucas Fowler, 23, and his US girlfriend Chynna Deese, 24, were found dead on the side of the Alaska Highway, 20 kilometres south of Liard Hot Springs, in British Columbia, a week ago (between 4pm July 14 and 8am July 15).

On Tuesday, the Royal Canadian Mounted Police raised fears the couple might be victims of a murderous crime spree in northern British Columbia by declaring it was possible the killings are linked to the discovery on Friday of another dead man, two missing teenagers and a torched Dodge pickup truck.

“It’s possible,” RCMP Corporal Chris Manseau said.

Lucas Fowler and Chynna Deese, who were murdered in a remote part of Canada. Photo: AAP

“We have two complex and dynamic ongoing investigations that are happening in northern BC.”

The sketch of the man police want to talk to. Photo: RCMP

The RCMP released a sketch on Tuesday of the man who was seen by a passerby talking to Mr Fowler on a remote highway in northern British Columbia a week ago.

The man who was seen talking to Mr Fowler was described “as Caucasian with darker skin and dark hair”.

“He’s shorter than Lucas who stood six foot three (190 centimetres) and has a possible beard or glasses,” RCMP Sergeant Janelle Shoihet said.

Lucas Fowler’s father fronted the media to help plead for information about the mysterious deaths of the Australian tourist and his American girlfriend on a lonely Canadian highway.

BREAKING: This is the first video we’ve seen of Lucas Fowler and his Gf Chynna on their road trip… hugging at a petrol station in Fort Nelson on July 13. Full video @TheTodayShow shortly.@9NewsAUS pic.twitter.com/HuD8TsSqAq

— Alexis Daish (@LexiDaish) July 22, 2019

Stephen Fowler, a senior NSW Police officer, said his family and the family of Lucas’s girlfriend, Chynna Deese, were shattered at the killings.

“We’re just distraught. This has really torn two families apart,” he said on Tuesday.

“It is a love story that’s ended tragically. It really is – it is the worst-ever love story.”

“I may be an experienced police officer but I’m standing here as the father of the murder victim,” Chief inspector Fowler, of Sydney’s north-west Hills district, said.

“We’re just distraught.”

The RCMP originally played down speculation of a serial killer or crime spree linking the murders of Mr Fowler and Ms Deese to Friday’s discovery 470 kilometres away of the body, missing teenagers and the burning Dodge pickup.

“We just know more about both incidents,” said Corporal Manseau, explaining the alarming change.

The two missing teenagers are 19-year-old Kam McLeod and 18-year-old Bryer Schmegelsky from Port Alberni, BC.

The RMCP said Mr McLeod and Mr Schmegelsky were driving the Dodge pickup but have not been in contact with their family for several days.

Sergeant Shoihet said investigators are not treating the teenagers as suspects but want them to contact authorities.

“We’re hopeful they alive,” Sergeant Shoihet said.

The body found two kilometres from their pickup was not one of the teens’, but a bearded man aged 50-60.

The RCMP have been unable to identify the dead man and also released his sketch.

-with AAP