CANBERRA, Australia -- An Afghan soldier who shot dead his Australian mentor at a patrol base and then fled was probably an insurgent who had infiltrated the Afghan army ranks, an Afghan colonel said Wednesday.

Army Lance Cpl. Andrew Gordon Jones, 25, was shot four times by his killer at a forward patrol base in the Chora Valley in Uruzgan province on Monday, Defense Minister Stephen Smith said.

The killer's motive has not been released. He was being hunted by his Afghan National Army colleagues, officials say.

The head of operations for the Afghan National Army's 4th Kandak (Battalion) in Uruzgan, Col. Abdul Qahar, said an investigation would likely determine that the killer was an insurgent.

"The cunning enemies of Afghanistan want to get inside the ranks of the Afghan National Army and police," Qahar told Australian Broadcasting Corp. radio.

"So I think that he was one of those enemies inside our ranks who carried out this action," he added.

Smith said the soldier had been training with his Australian mentors for months. Smith suspected the reason for the death was an earlier argument between the Afghan and his victim. Smith did not say what the argument was about.

"We don't believe we're dealing with an impostor or an infiltrator," Smith told the ABC.

The death was one of two Australian fatalities on Monday that brought Australia's toll in the conflict to 26.

The second fatality was 27-year-old Lt. Marcus Sean Case, who was killed when a Chinook helicopter crashed while on a resupply mission 56 miles (90 kilometers) east of the Australian base in Tarin Kot in Uruzgan.

Smith said an investigation had ruled out enemy fire as the cause of the crash.

Australia has 1,550 soldiers in Afghanistan with a primary focus on training an Afghan National Army battalion to take responsibility for security in restive Uruzgan.

Australia, the largest military contributor to the U.S.-led alliance in Afghanistan outside NATO, plans to start withdrawing troops once the Afghan battalion is fully trained as early as next year.

Australia joined the U.S.-led war against the Taliban in Afghanistan in 2001, but opinion polls show that public support for the campaign has waned over the years as the death toll has mounted and victory proved elusive.