A manhunt is underway after an American woman and her Australian boyfriend were murdered while on a road trip through Northern British Columbia. Canadian police aren't saying how Lucas Fowler and Chynna Deese died, but they are investigating these deaths as homicides. Police in Fowler's native Australia said it appears they were both shot.

The deaths happened near Canada's notorious "highway of tears" – a remote stretch of road where at least 18 women in the last 40 years have disappeared or been found murdered.

"At this point we have nothing to indicate that their deaths are linked to any other active and ongoing investigations," said Janelle Shoihet of the Royal Canadian Mounted Police. "It's not yet clear whether Lucas and Chynna were targeted, or if this was an opportunity, a crime of opportunity."

Stetson Deese describes his 24-year-old sister Chynna as a free spirit who traveled the world, oftentimes with her boyfriend.

"She was really adventurous, really outgoing," Stetson said. "They were both extremely nice, both very optimistic and also spontaneous."

The couple's bodies were found last Monday by their van on a highway near the Liard Hot Springs, a popular tourist destination.

Curtis and Sandra Broughton say they met Deese and Fowler shortly before their deaths. The Broughtons pulled over to help the couple with their broken-down van and left when it appeared they knew how to fix it.

"We were there to help them … but yet that same day the complete opposite of humanity hurt them like that and killed them so tragically," Sandra said.

Stetson Deese said he always felt his sister was safe traveling, especially while with Fowler, and believes the couple was going to get engaged soon.

"It's like a nightmare that we'll never wake up from," he said.

Fowler's father is a police officer in Australia and he and other officers are now in Canada. Deese's brother told CBS News her passport was found in the van, but it appears the couple's other ID's may be missing.