In Afghanistan, who is in charge?

>>> command headquarters of the international security assistance force in kabul. we have been traveling through this country all week trying to take measure of this war, another insider attack has left a u.s. soldier and an american civilian dead. that brings the death toll to 2,118. so far this year, 267 have been killed. 31 of those killed at the hands of afghan police and military members. insider attacks. tonight against the backdrop of next month's presidential election, we're beginning a special look at a region at the brink and the foreign policy challenges that america is facing here in syria and afghanistan. richard engel , you have more on that attack today?

>> reporter: cooperation between u.s. and afghan forces is the mission now. it is the american exit strategy . but this recent spate of insider attacks seems to show that the exit strategy isn't working. even the scene of the aftermath isn't encouraging. after gang security forces investigating themselves, a training mission where who's in charge, who can be trusted is now unclear. last night an afghan soldier known here as a & a walked up into a group of an unsuspecting group and opened fire.

>> in an ensuing exchange of fire, three a & a personnel are reported to have died. we deeply regret the loss of life in this tragic incident.

>> reporter: tragic certainly but some commanders have suggested it's cultural misunderstanding. it's the 57 death like this this year. the commanding general here spoke with lester about it.

>> we're going to work as hard as we possibly can around the clock to understand the problem and i think we have a good grip on it now and to work closely with our afghan partners to eliminate this threat.

>> reporter: but it's hard to defend against an attack from within. the u.s. mission now is to closely partner with afghan forces. lieutenant phillips is in close contact with afghans every day, training them and providing weapons on air support .

>> how do you know you can trust the afghans you're working with?

>> you don't know. that's why we make friends with them as best as we can, with all the other afghan units that we go to. but you don't know whether they're with us or they've got an ulterior mottive.

>> reporter: when he meets with his counterparts, it's without his flack jacket and helmet, to show trust. but trust with backup, isn't real trust. there's supposed to be two more years of this, but if the insider attacks continue, it's hard to see how that's possible. u.s. troops say they understand the risks of war and that they're willing to take these risks, but not to be shot in the back by their allies. allies we're paiding to train.