By NBC's David Murphy
31 U.S. troops in Afghanistan have now died at the hands of Afghan security forces – men and women that are supposed to be American allies.
The latest so-called “green-on-blue” attack killed a U.S. service member and an American contractor over the weekend.
Insider attacks are blamed for 20% of all U.S. military deaths in Afghanistan this year – a trend the U.S. Commander there, General John Allen, says makes him “mad as hell”.
38 “green-on-blue” attacks have been reported so far in 2012 – nearly as many as in the five previous years combined.