RCMP say they may never know what caused an SUV carrying five people from Prince George, B.C., to crash head-on into a semi-trailer five kilometres outside McLeese Lake on Thursday.

Matt Altizer, 40, his wife, Leah and their two children — Jonathan, a Grade 8 student, and Emily, a Grade 6 student — died in the crash.

Heather Kress, a passenger in the Altizers' SUV, was also killed. It's believed she was Matt Altizer's sister.

"Everyone was related through family," confirmed RCMP Cpl. Madonna Saunderson.

The five were on their way to Vancouver to watch the Davis Cup tennis competition, RCMP said, when their SUV crossed the centre line of Highway 97, about 60 kilometres north of Williams Lake.

The SUV crashed head-on with a semi-trailer and burst into flames. RCMP Inspector Eric Brewer said they may never determine what caused the crash because of the extensive damage to the SUV.

"That's fairly common where there's a significant fire and the vehicles are basically totally destroyed. If you don't have other physical evidence, you're kinda left hanging as to what may or may not have occurred," said Brewer. "It can be hugely challenging, because really we don't know if there is a mechanical issue with the vehicle, we'll never know that."

Initial reports were that road conditions at the time of the crash were bare and dry with good visibility. Police said on Friday that further investigation revealed the highway was bare, but wet.

The driver of the semi was hospitalized in Williams Lake with minor injuries.

"He was fairly traumatized by the whole circumstance. You know he gets out of his truck. This is what he is encountered with. You know it's not something most people get to see every day," said Brewer.

Prince George mourns loss of Altizer family

"There's just a real sense of loss here, in Prince George," said CBC reporter Brady Strachan.

"Everybody knows everybody, or knows someone who knows someone here. People are just shaking their head about such a tragedy, to have taken so many lives."

Altizer had been the IT manager at the Prince George Citizen for over a decade, said interim publisher Coleen Sparrow.

A memorial set up by neighbours outside the Altizer home in Prince George. An inscription reads 'For my dear friend Leah and her family. Miss you already, much love Cheryl Brown & family' (Brady Strachan/CBC)

"Quite honestly, right now, I can't even believe it's for real," said Sparrow through tears. "It's just a huge loss for this community."

"The kids were at school and they were just his pride and joy. We'll miss him," said Sparrow.

Travis Shaw also knew Altizer.

"It's a tragic loss, any loss is awful. But there's a family is involved and more than one person its so traumatic and hits the community really hard," he said.

Police said it's too early to tell whether alcohol or speed were factors, and officers continue to investigate.

With files from the CBC's Brady Strachan