Image credit: 4th Infantry Division Public Affairs Office

ABC News' Mike Boettcher reports from Kunar province, Afghanistan:

The war in Afghanistan - the nation's longest - has now reached a grim milestone: 2,000 Americans killed in the country.

The 2,000th death - suspected to be an insider attack by Afghan forces, or "green on blue" - was reported Saturday in eastern Afghanistan. During the first nine months of this year, 254 members of the U.S. military lost their lives in Afghanistan.

They are soldiers like Army First Lt. Alejo Thompson, a husband and father of two from Yuma, Ariz. He was killed in May by an Afghan soldier he was mentoring.

Sgt. Joshua Danison witnessed the insider attack and gave ABC News the first eyewitness account.

"We woke up one morning to some gun shots being fired," he said. "At the time, it was very chaotic. It was an Afghan National Army soldier. We seen the shooter actually walk up to Lt. Thompson and shoot him."

Thompson's killer escaped after the attack and was shown in a Taliban video receiving a hero's welcome.

Now the men who trust the Afghan security forces with their lives are wrestling with the betrayal.

"There were comments made that we don't trust them. We don't want to be around them. We don't want to partner with them anymore," Danison said. "But to overcome that, we have to look at it as a mission and it is in our character - from the time we come out of basic training - to accomplish missions."

But for the troops of Task Force Red Warrior, Thompson's unit, worry remains. Uneasiness is a part of the mission now and will likely be until the last American troops leave the country in 2014.