A mysterious man, another body and an infamous highway: What we know about murder of Lucas Fowler and Chynna Deese.

-With AAP.

Australian tourist Lucas Fowler and his American girlfriend, Chynna Deese, were smiling and relaxed as they sat on armchairs next to their broken-down Chevrolet van on the side of a remote Canadian highway.

It was about 3.20pm on Sunday and Fowler, 23, from Sydney, and Deese, 24, from North Carolina, were enjoying life despite being stranded about 200 kilometres from the state border with Yukon.

Hours later they would be dead, their bodies lying near their conked out blue 1986 van, still broken on the roadside.

The couple that was so full of life – and with an insatiable desire to travel the world – had been shot dead.

Their injuries were so severe it took police three days to identify the couple. Deese's family was told that due to the nature of her injuries, they would not be able to have an open casket at her funeral.

The hunt is on for their killer or killers and police are investigating whether they were targeted murders or crimes of opportunity.

The bearded man.

Police have released a composite sketch of a man they believe to be one of the last people to speak to Fowler and Deese on the evening of July 14.

Royal Canadian Mounted Police (RCMP) spokeswoman Corporal Janelle Shoihet said the man - who was described as Caucasian, with darker skin and dark hair with possible beard or glasses - was not a suspect, but a person of interest.

The man may be associated to an older model Jeep Cherokee with a black stripe on the hood and a black light/bull bar with small, covered lights, police said.

A road worker witness claimed she saw a mystery bearded man arguing with Fowler and Deese before their murders.

Alandra Hull told 9News the man was arguing with the couple while his vehicle was parked down the road on the Canadian highway where the couple was stopped.

She said the man was standing in the middle of the highway, while Fowler and Deese stood beside their van.

"If you just get a bad feeling, and that's what I had, you just don't stop," she said.

A third body and the missing teenagers.

Less than a week after Fowler and Deese's bodies were discovered, a third body was found on nearby Highway 37 near a burning Dodge pick-up truck south of the Stikine River Bridge on Highway 37 in northern British Columbia and now police are appealing for information about two missing teenagers.

The truck had a distinctive sleeping camper attached, and was found approximately 470km away from where Fowler and Deese were discovered.

The Dodge was driven by 19-year-old Kam McLeod and 18-year-old Bryer Schmegelsky from Port Alberni, British Columbia, and police have confirmed the body was not either of them. It has not yet been identified.

BC RCMP are asking for the public’s assistance in locating 19 yr. old Kam McLeod & 18 yr. old Bryer Schmegelsky who have not been in contact with their family for the last few days & were driving the vehicle found on fire 50 km south of Dease Lake. https://t.co/achxhv1Dj0 pic.twitter.com/bFrqKucerL

— BCRCMP (@BCRCMP) July 22, 2019

The RMCP said McLeod and Schmegelsky "have not been in contact with their family for the last few days".

They said it was unclear how the dead male near the Dodge might be connected with the vehicle fire or the two missing teens.

Originally, Canadian authorities talked down speculation that a serial killer could be on the loose but Corporal Shoihet has acknowledged their could be a link:

"It is unusual to have two major investigations of this nature and we recognise the possibility that these could be linked," she said.

Deese's family fear the killer may strike again.

"I don't think it's a serial killer," her father, Dwayne, told the Charlotte Observer. "I think of someone who has been convicted of violent crimes before, someone on drugs.

"That fits the profile better. What worries us is that person is still on the loose and they have a head start.

"This is going to happen again. There needs to be some kind of a warning system in place for tourists."

Lucas and Chynna's last moments.

It was a Good Samaritan Canadian couple, Curtis and Sandra Broughton, who were among the last to see Fowler and Deese alive.

The Broughtons were heading home along Alaska Highway after visiting Liard Hot Springs on Sunday at 3.20pm when they saw the Chevrolet van on the side of the road.

Curtis Broughton is a mechanic and presumed the young couple relaxing on fold-out chairs needed help.

"Obviously their van had broken down, but they were still happy and smiling," he told AAP on Friday. "They were having lunch or a bit of a meal when we pulled up."

Broughton soon realised the Australian lad had it all under control.

"He seemed like he had everything diagnosed properly.

"The vehicle was flooded out and they were going to try and get it going again until they could get the parts they needed."

The Broughtons got back in their car and drove home.

Broughton said he did not notice anyone suspicious in the area but, days later, was horrified to learn from social media the couple they stopped to help had been murdered.

NEW PHOTOS: Here’s a look at the van recovered at the scene where Lucas Fowler and Chynna Deese were found. ⁦@GlobalBC⁩ pic.twitter.com/ZbByI4HUPJ

— Nadia Stewart (@nadia_stewart) July 21, 2019

The Highway of Tears myth.

Despite the reporting of many international media, Fowler and Deese were murdered more than 1000 kilometres north of British Columbia's infamous 'Highway of Tears'.

Since 1970, up to 50 women (mostly Indigenous Canadians) have been murdered or have disappeared on this 724 kilometre stretch of the highway that stretches across central British Columbia.

Along the road there are warnings not to hitchhike for fears a serial killer lives in the area.

"There's some indications from international media that they may be related to other crimes that occurred in different parts of the province. We're talking about completely different geographical areas," RCMP sergeant Janelle Shoihet told Alaska Highway News. "There's nothing to link those two at this point."

In 2015, the Canadian Government launched a national inquiry into the death of Canada's Aboriginal women, of which the Highway of Tears was a part.