KABUL, Afghanistan -- man in an Afghan uniform shot and killed three American troops Friday morning in southern Afghanistan, the U.S. military command said, in the third attack on coalition forces by their Afghan counterparts in a week. The Taliban claimed the shooter joined the insurgency after the attack.

According to the Dept. of Defense, killed in the explosion were:

	Army Command Sgt. Maj. Kevin J. Griffith, 45, the senior enlisted soldier of the 4th Brigade Combat team, 4th Infantry Division from Fort Carson, Colorado.
	Army Maj. Thomas E. Kennedy, 35,  of West Point, New York.
	Air Force Maj. Walter D. Gray, 38, of Conyers, Georgia.

The attack is the latest in a rising number of so-called "green-on-blue" attacks in which Afghan security forces, or insurgents disguised in their uniforms, kill the U.S. or NATO partners who are training Afghans to take over once most international forces leave in 2014. Compared to the 21 attacks this year that killed 30 foreign troops, there were 11 such attacks and 20 deaths in 2011. And in 2007 and 2008, there was a combined total of four attacks and four deaths.

So far this year, at least 21 similar attacks -- in which Afghan forces or insurgents disguised in Afghan uniforms have turned their guns on international troops -- have killed 30 coalition service members, according to an Associated Press tally.

Friday's shooting took place in Sangin district of Helmand province, said U.S. military spokeswoman Maj. Lori Hodge. She gave no further details and said the military were investigating.

Exactly what happened in the attack was unclear, and there were conflicting accounts. Sangin Gov. Mohammad Sharif said the shooting happened at a police checkpoint after a joint meal and a security meeting, but an Afghan army commander, Farooq Parwani, said that the attack happened on a U.S. base.

The U.S. military said it was also not clear whether the attacker wore an Afghan army or police uniform.  Read more at Fox News.