( CNN ) -- In late November , U.S. soldiers were supervising artillery training for Afghan troops in Nangarhar province , close to the Pakistan border . After they climbed a ridge to assess the impact area , an Afghan border policeman suddenly turned on them and opened fire . Seconds later , six Americans and their assailant were dead . It was one of the deadliest incidents of what the military call `` green on blue '' -- a euphemism for attacks by members of the Afghan security forces on their allies in the international force known as ISAF . They have risen sharply over the past year , raising fears that the Taliban are infiltrating the Afghan National Army and police force . But military analysts and intelligence officials say the reality is more complicated . The Taliban said they had recruited the border policeman , but his real motive was never established , and it transpired that he 'd had an argument with his father over his choice of bride . In an even deadlier incident , an Afghan air force officer shot dead eight U.S. Air Force personnel and a U.S. contractor at Kabul International Airport in March . But there is little to suggest he was a Taliban infiltrator either . According to a NATO analysis seen by CNN , `` green on blue '' attacks have killed 52 U.S. and allied soldiers since 2005 , many of them in the past year . The analysis concludes that combat stress provoked 36 % of the attacks , even if the Taliban subsequently claimed responsibility . In 23 % cases , an Afghan soldier had been persuaded by the Taliban to carry out an attack -- but the motive in an additional 32 % of cases was unknown . A senior intelligence official involved in NATO 's training program told CNN that `` battlefield conditions and frequent deployments are the leading causes '' of such incidents . `` When you are in the mountains for months , you 've just had enough , '' he said . The official said cultural differences over the handling of weapons and the attitude of western soldiers to Afghan women can exacerbate tensions . Coalition officers often reprimand their Afghan colleagues for lax control of weaponry , and that can breed resentment . Some incidents defy explanation . In late 2008 , an Afghan soldier liked and relied upon by his U.S. comrades in Wardak province suddenly turned his gun on them and shot two dead before fleeing . He was never found . For the Taliban , infiltration is a cheap tactic and one they boast about . Beyond the incidents , it sows mistrust and apprehension on a wider scale among troops who should be cooperating . As the struggle against the Taliban reaches its climax , military analysts say a growing `` insider threat '' could be disruptive . For example , after three German soldiers were shot dead by an Afghan soldier this year , some of their comrades refused to go on patrol with their Afghan counterparts , according to the magazine Spiegel . Earlier this month , Lt. Gen. William Caldwell , the commander of the NATO training mission , admitted recent incidents `` erode the hard-earned trust that is required for an effective partnership . '' But the senior intelligence official told CNN that the insurgents ' efforts to infiltrate Afghan security forces had not so far yielded much of a dividend and was n't anywhere near the top concerns facing ISAF . He pointed out that the 52 coalition deaths caused by Afghan security personnel were a small percentage of nearly 2,000 `` hostile fatalities '' between 2005 and 2011 . Other estimates are higher -- but include killings by Afghan contractors and incidents of `` impersonation , '' where an insurgent launches an attack disguised as a soldier or policeman . Police and military uniforms are easy to come by in Afghanistan . In one incident this year , a soldier passed on a uniform and identification to a suicide bomber who attacked a military hospital in Kabul . Six people were killed . One reason `` green on blue '' attacks have risen is because there 's more interaction between U.S. and Afghan soldiers as the latter take on more responsibility . There are about 290,000 Afghans in uniform . The number of joint military patrols has increased drastically , exceeding 10,000 last year . Among measures being taken to prevent such attacks , commanders are told to ensure Afghan troops who 've been in combat are getting adequate rest ; and coalition officers are being taught to watch for warning signs . More U.S. counter-intelligence officers are being deployed to Afghanistan to help with screening recruits and reviewing the profiles of serving soldiers . The recruiting process now includes eight steps . A recruit 's application must be supported by two letters from community elders . Gradually a nationwide database of Afghan security forces is being compiled based on biometric information . Some fifty Afghans at the Interior Ministry , helped by the FBI , are populating the database . The objective is to screen out anyone with a criminal record or links to the insurgency . The process of vetting and biometrically enrolling the records of nearly 300,000 Afghan security forces will be completed this year , according to a NATO training mission document compiled in February .
These include fears that the Taliban are infiltrating the Afghan forces . Military analysts and government officials say the issue is more complicated . An analysis concludes that psychological stress caused 36 % of the attacks .