Story highlights Greg Buckley Jr. was gunned down by the forces he had been training

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

The phenomenon is known as "green-on-blue" and is disturbingly more frequent in Afghanistan

Buckley warned superiors an insider attack could happen, his father says; CNN could not confirm

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 was 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.

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"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 say 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 each other, yelling until a group of officers separated them, 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.

The phenomenon is known as "green-on-blue," due to a color-coding system used by NATO. It has become disturbingly more frequent in Afghanistan, with more than 50 NATO troops killed this year by local forces, the first time that's happened in a single year in the U.S.-led war.

Last year, 35 people died in such insider attacks, and even less the year before, according to NATO figures.

The killings have prompted suspensions of training new recruits while eroding the trust between NATO and its Afghan allies.

The gunman involved in Greg's death attacked from inside his outpost and killed two other fellow Marines, his dad said.

But Greg Sr. said his son had informed his superior officers that "one day they are going turn around and turn those weapons on us."

CNN cannot independently confirm that Greg informed superior officers. Calls and emails to NATO's International Security Assistance Force in Afghanistan were not immediately returned.

More than a decade after the war began, the Buckley family is now struggling to cope with the loss of its eldest son. More than 2,000 other U.S. service members have been killed in Operation Enduring Freedom.

"It's not really day-by-day," said Greg's mother, Marina Buckley. "It's more minute-by-minute."

Back in Oceanside, the Buckley family on Friday attended the town's first home football game, where their fallen son had intended to watch his youngest brother play varsity for the first time.

"Greg was supposed to be home for this game," said Justin, 17, who wore the number 30 on his back, Greg's old basketball number.

"I would tell him I love him and I miss him."

The senior running back, who donned a camouflage jersey along with his team, broke to the outside on Friday for a 25-yard score that helped cement the Sailors' improbable second-half comeback against top-ranked East Meadow.

After crossing into the end zone, Justin raised his hand to salute -- honoring his fallen brother.