Police searching for the killer in the unsolved double murders of Australian man Lucas Fowler and his American girlfriend Chynna Deese have described a "person of interest" and revealed how the couple died.

Royal Canadian Mounted Police held a press conference today, a week after the couple were found on the side of Highway 97 in British Columbia.

Police said the pair were "victims of gun violence" after earlier revealing their bodies were "unrecognisable".

Police again dismissed the notion that a serial killer is on the loose after a third victim was found on the same stretch of highway days later.

But they did confirm they are seeking to speak with a man seen arguing with Fowler, 23, on the night before he was murdered.

"Today we're asking for the public's assistance in identifying a man seen speaking with Lucas on the evening of July 14 on Highway 97," a RCMP media spokeswoman said.

"He is caucasian, has dark skin and dark hair, is shorter than 6'3" and has a possible beard and glasses.

"He was driving an older model Jeep Cherokee with a black stripe on the hood and was believed to have been travelling southbound."

Officers at the press conference — which was attended by Lucas's father, NSW Police Inspector Stephen Fowler — were peppered with questions from frustrated members of the local media about why there has been so little information about the investigation.

"Why can't you give us an indication of how he died?" one reporter asked in relation to the third victim, found near a burnt-out car.

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

"The public is wondering what is going on. They're not getting answers from you. Why can't you give us an idea about whether there is a linkage (between the three deaths)? Is there a serial killer? People have the right to know."

But police would not go into detail. They would not confirm whether the third victim was murdered or how he died.

Police today released a composite sketch of the man they wish to speak to, as well as CCTV footage of Fowler and Deese arriving at a petrol station on July 13. They said they are going through "hours" of CCTV footage and that the investigation is painstaking.

Fowler and Deese were found a short distance from the blue 1986 Chevrolet van they were travelling in on Monday morning.

The third body was found a short distance up the road next to a burnt out vehicle. But in a bizarre twist revealed yesterday, police now say the man was not the vehicle's owner and that two teens travelling in that vehicle are missing.

In a statement, the Dease Lake Royal Canadian Mounted Police said they have been unable to locate Kam McLeod, 19, and Bryer Schmegelsky, 18.