Deployed to a volatile outpost in southern Afghanistan where U.S. Marines routinely face a mix of skirmishes and hidden explosives, Greg Buckley Jr. sensed that an attack was imminent.

And he knew that it would come from within.

The 21-year-old Marine was posted to Garmsir in Helmand Province, where he was training local security forces as part of NATO's planned withdrawal in 2014.

It was during a static-filled phone call to his father over the summer that the Long Island native mentioned a run-in he had with an Afghan trainee while on guard duty.

The encounter marked the first in which the Buckley family's eldest son seemed to sense something was wrong, according to what he told his family in phone conversations and a letter.

"The guy turned around and said to Greg, 'We don't want you here. We don't need you here,'" his dad said.

"Greg turned around again and said, 'Why would you said that?'" according to Greg Buckley Sr.

But the trainee apparently wouldn't relent, repeating the phrases for hours over the course of a night in which the young Marine was on guard watch.

"Greg said, 'I thought I was going to lose my mind,'" his father said. "Pitch black out, and all he kept saying over and over again is, 'We don't want you. We don't need you. We don't want you.'"

"It was just tormenting for him."

The two men then finally confronted, yelling until a group of officers brought them apart, he told his father.

"One of his superiors came over and had Greg apologize to the guy," said the elder Buckley.

The 21-year-old agreed and extended his hand, but the man refused.

About a month later, Greg phoned his father again.

"He told me if I have to stay here until November... I'm not going to come home."

Greg also asked his father to prepare to tell his mother and his two younger brothers that he'd be killed.

"I don't understand," his father said. "Out in the field?'

"No, in our base," Greg replied.

On August 10, 2012, Greg Buckley Jr. was gunned down by the very forces he had been training, just days after learning that he was to head home early.

"It was only two days he had left there in Afghanistan," his father told CNN.