A Halifax Regional Police officer died after his minivan collided with this truck on Wednesday. (CBC)

A Halifax Regional Police officer was killed in an accident on the Trans-Canada Highway near Truro early Wednesday morning.

Const. Tony Carlisle, 41, was off duty when he was driving a minivan that collided head-on with a tractor-trailer on Highway 104, Colchester County RCMP said.

The van was travelling west on the four-lane, divided highway when it crossed into the eastbound lane, RCMP said. The crash happened about two kilometres from the interchange with Highway 102.

The driver of the tractor-trailer was not injured.

Colchester County RCMP Const. Al Affleck said investigators don't know why Carlisle was on the wrong side of the road.

"Everybody in the policing community are asking that question. And right now, I cannot answer that. The investigators cannot answer that," he said.

"What we want to do now is retrace the evening, the last few hours of where the individual was and to get some type of insight into what may have happened."

Weather conditions were wet and foggy at the time of the accident, the RCMP said. Visibility at the time was about 300 metres.

In a statement, Halifax police said Carlisle was hired by the department in May 1995. He was most recently assigned to the patrol division and also spent six years in the criminal investigative division.

"This is a tremendous loss not only for the extended police family but for the community as a whole. It’s devastating to lose one of our own, especially as this time of year," said Halifax police Chief Frank Beazley. "Our thoughts and prayers are with the entire Carlisle family."

An autopsy will be performed.