Sgt. J.P. Huling, a Marine from Ohio, was killed this month in southern Afghanistan.

It wasn't a roadside bomb or a Taliban sniper that killed him. It was another sergeant — an Afghan soldier known as Sgt. Zabitollah, who like many Afghans went by one name.

It was a grim coincidence that brought these two sergeants together on May 6, a Sunday afternoon, at a mud-walled compound along a desolate stretch of road in a remote corner of Afghanistan.

Huling wasn't even supposed to be there. He wasn't scheduled to leave his base in Southern California until the fall, but he was called up early. Several other Marines in his bomb disposal unit had lost their lives already. They needed Huling to do the job.

In Afghanistan, Sgt. Zabitollah had just visited his family near Kabul.

Both men were married, in their 20s. Both had drifted away from school and joined the military.

Huling took culinary courses at a community college. Sgt. Zabitollah made it to the ninth grade, then left his family farm for the Afghan army.

"Pretty much he was a good sergeant. We were not suspicious of him," said Col. Narouz, who was Zabitollah's battalion commander. "He was a great man. He used to work shoulder to shoulder with Marines."

On the American side, Huling's friends say he joined the bomb disposal unit because he wanted to make a difference.

"Those closest to him characterize him as honest, as a stand-up guy," says Maj. Gen. Mark Gurganus, who commands all Marines in Helmand. "He loved his wife dearly, because he talked about her all the time.

No Clear Answers

So far, no one has a clear answer for why the Afghan sergeant turned his AK-47 on Huling, shooting him in the stomach and killing him.

Zabitollah was quickly shot dead by the Americans. An investigation by Afghans and the Marines is under way.

What is certain is that these killings are on the rise. Last year, 35 U.S. and NATO forces were killed by Afghans wearing Army or police uniforms.

This year, 22 American and coalition troops have been killed. Three of them have been killed in the past week alone.

"The threat within right now is worrying me," says Lt. Col. Michael Styskal, who commands Marines in the central part of Helmand province. "And the Marines, they know what the threat is, not so much on the outside — there could be a threat on the inside."

Styskal speaks from painful experience: One of his Marines was killed earlier this year by a man wearing an Afghan army uniform.

New Precautions

So American troops are taking precautions. Some are secretly wearing light armored vests under their uniforms whenever they meet with Afghan soldiers or police at their camps.

Now, whenever Afghans and American troops get together, one American is dubbed the "guardian angel" — with an M4 assault rifle at the ready.

For their part, the Afghan officers carefully question their soldiers and police when they return from leave. That's when officials fear they may have been persuaded to support the Taliban.

Styskal says his gut tells him that about half the coalition deaths are from Taliban infiltrators.

"There is a threat of infiltration out there .... The Taliban have even publicly said we need to get into the [Afghan security forces] and stir up the mess," he says.

The Taliban claim credit for all the deaths of coalition troops at the hands of Afghans in uniform.

Heat-Of-The-Moment Disputes

But a recent Pentagon report says most of those troops killed were not the result of Taliban penetrating Afghan security forces. Instead, the report says, the majority are just personal disputes that get out of hand.

That may explain the circumstances surrounding Sgt. Huling's death.

The head of the Afghan military in Helmand province, Gen. Muhammed Ali Shuja-e, thinks there may have been angry words between the two sergeants.

"Some sort of argument may have happened, so this was not totally clear," he says.

Afghan officials say the Afghan forces were searching inside a compound, while the Marines set up security outside. Then, Zabitollah stepped out of the compound and walked toward Huling, who was down on one knee.

"The Marine called him and said don't step out — maybe you'll get shot by the enemy. The Marine yelled two or three times," says Narouz, the Afghan battalion commander.

Suddenly, the Afghan sergeant turned and shot Huling. He later died at an American hospital.

Searching For Clues

Afterward, the Afghans found Zabitollah's journal, which included his father's phone number. An Afghan soldier called Zabitollah's father.

"He said, 'My son was a bright man, there was no connection between him and the Taliban.' He didn't accept the accident happened," says Narouz.

Zabitollah's body was returned to his family. His colonel admits they may never know the truth about why he killed Huling.

"Since he's dead, I can't interrogate him," the Afghan colonel says.

Huling's funeral was held in his small Ohio town on what would have been his 26th birthday. He was awarded a Purple Heart. And Gen. Gurganus is writing a letter to Huling's wife, Priscilla.

"It's really to offer condolences. And we share in the grief of this tragedy. My guarantee to her is we won't forget him," he says.

Gurganus and others say the Afghan commanders also share in the grief and are heartbroken by what happened.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.