The investigation into the mysterious shooting deaths of Australian backpacker Lucas Fowler and his American girlfriend has taken a twist, amid the discovery of an unidentified body and the disappearance of two teens.

Canadian authorities have called for the public to “remain vigilant” following the spate of strange incidents in northern British Columbia in the past week.

It comes as clues emerged into the killings of New South Wales man Mr Fowler, 23, and his partner Chynna Deese, 24, who were found dead on July 15 just days into their road trip adventure. 

Investigators have in recent days spoken to a witness who saw a bearded man in a heated exchange with the young couple in the middle of the Alaska Highway.

Australian detectives have flown over to help in the case, along with Mr Fowler’s father who is a senior NSW policeman.

Peter Fowler, a chief police inspector, told Nine News as he stopped at Los Angeles airport that the family had been “crushed” by the loss of his son and girlfriend.

The bloody bodies of Mr Fowler and Ms Deese were found near the blue minivan they had been driving on the Alaska Highway, 20 kilometres south of Liard Hot Springs.

On Saturday, police found another man dead, at a rest stop close to a burnt-out red and grey Dodge pick-up truck.

Police have since confirmed the man was not the Dodge’s owner – and that the two teenagers who were travelling in the vehicle were missing.

In a statement, the Royal Canadian Mounted Police said they had been unable to find Kam McLeod, 19, and Bryer Schmegelsky, 18.

RCMP released photos of Kam McLeod (left) and Bryer Schmegelsky. Photo: RCMP

The RMCP said Mr McLeod and Mr Schmegelsky had “not been in contact with their family for the last few days”.

The RMCP has repeatedly fended off speculation there is a serial killer in the area, and have not publicly linked the incidents.

On Sunday, however, it acknowledged “growing community concerns about the ongoing homicide investigations in northern BC”.

“At this time, investigators are sharing information and police would like to ensure awareness around both investigations,” RCMP spokesperson Dawn Roberts said.

“The RCMP would ask the public to continue to take any general safety precautions and remain vigilant at this time.

“We also remind travellers to share your plans with family and friends, establish check-in times and notify someone if your plans change.”

The RCMP was also forced to apologise after an embarrassing mistake in which it live-streamed a media conference about the murders of Mr Fowler and Ms Deese with an augmented reality “cat filter”.

Sergeant Janelle Shoihet, who spoke at the conference, blamed “technical difficulties” after re-recording her statement. The RCMP said on Twitter that it had accidentally left on “an automatic setting.”

The B.C. RCMP are giving a press conference on the two people murdered on the Alaska Highway, and they have the cat ear filter on. pic.twitter.com/j8GvkvKA4u

— Tyler Dawson (@tylerrdawson) July 19, 2019

The RCMP said the burned-out Dodge was found near Dease Lake, about 470 kilometres from where Mr Fowler and Ms Deese were found.

Police confirmed the body discovered near Dease Lake was not Mr McLeod or Mr Schmegelsky.

The teenagers were travelling through BC to visit Whitehorse in the Yukon territory to look for work. The RCMP said it was not clear what their travel plans were.

“Kam and Bryer have periodically connected with family and friends over the past week and it is possible that they are now in an area without cell coverage,” Ms Roberts said.

“However, we have found their vehicle and have not been able to locate either of them at this time.”

She asked Mr McLeod and Mr Schmegelsky to get in touch with police urgently. Anyone who had seen or spoken to them should also call police.

The RCMP said it was unclear how the dead man near the Dodge might be connected with the vehicle fire or the missing teenagers.

Mr Fowler and Ms Deese met backpacking in Croatia two years ago, fell in love and embarked on a Canadian road trip in the old Chevrolet.

Passing motorists saw the stranded couple alongside their broken-down vehicle.

Canadian couple Curtis and Sandra Broughton stopped to offer help about 3.20pm on Sunday.

Mr Broughton, a mechanic, said he spoke to Mr Fowler but the young Australian seemed to know how to fix the van, so the Broughtons left.

Perhaps the biggest clue came from a road worker, Alandra Hull, who told the Nine Network she saw Mr Fowler and Ms Deese on the highway the night before they were found dead, talking to a man who “kind of looked frustrated or something”.

That man was standing in the highway staring at Mr Fowler and Ms Deese.

Ms Hull is working with an RCMP artist to provide a sketch of the man.

-with AAP