KABUL — An Afghan border policeman turned his weapon on NATO soldiers during a training mission in eastern Nangarhar Province on Monday, killing six American troops before others gunned him down, U.S. and Afghan officials said.

The shooting took place in the remote Pachir Agam district near the border with Pakistan, according to Afghan Ministry of Interior spokesman Zemeri Bashary. Officials of the International Security Assistance Force, or ISAF, did not immediately release the identities of the victims.

The incident is the latest in which Afghan security forces have turned on their international trainers and mentors and it renewed questions about the trustworthiness of the Afghans as ISAF troops accelerate their efforts to transform them into professional police officers and soldiers.

Less than two weeks ago, NATO and Afghan officials meeting in Portugal renewed their pledge for an enduring partnership and declared that the Afghan forces would be ready to assume responsibility for the security of their country by the end of 2014.

“This is an issue of concern, but it cannot deter our partnership with ISAF,” Bashary declared.

ISAF and Afghan authorities said they had launched a joint investigation into the shooting to attempt to determine, among other things, whether the shooter was acting alone or had been recruited by insurgents to infiltrate the international forces. The Taliban claimed responsibility for the attack.

NATO is still investigating an incident in which two U.S. Marines were killed earlier this month in southern Helmand province, allegedly at the hands of an Afghan army soldier.

On July 20, an Afghan army sergeant got into an argument at a shooting range in northern Afghanistan and shot dead two American civilian trainers before being killed; another Afghan soldier was killed in the crossfire. In a July 13 attack, an Afghan soldier stationed in the south killed three British troopers, including the company commander, with gunfire and a rocket-propelled grenade in the middle of the night.

In November 2009, an Afghan policeman killed five British soldiers at a checkpoint in Helmand. And a month earlier, an Afghan policeman on patrol with U.S. soldiers fired on the Americans, killing two, the AP reported.

At the Adraskan National Training Center in Herat Province on Monday, American trainers said they are keenly aware of trust issues in training the Afghan forces. But Army Lt. Col. Mike King, who oversees the center, said “just knowing who you work with” is key to protecting themselves.

“If there’s a handful of bad actors in a group of people, do you judge them all by that handful?” he added.

Some Marines recently interviewed at Adraskan said they make it a point to stay vigilant.

“Especially when we’re on the range and they have weapons,” said Lance Cpl. Abel Hernandez, 26, of Riverside, Calif. “It crosses our minds.”

The Marines, as well as Italian and Polish trainers working at the center, stand close to the Afghan recruits on the range, often keeping one hand on them, so that if a recruit does turn with a loaded weapon, he can be neutralized.

“You don’t have to trust them, but show them respect,” said Sgt. Daniel Ford, the leader of the Adraskan team. “We don’t show that we don’t trust them.”

NATO trainers acknowledge that, given how quickly they are working to recruit and train new Afghan security forces, they are sometimes unable to effectively vet their Afghan students.

In the past year, the Afghan police force grew 27 percent to 120,500 officers, while the army grew 42 percent to about 138,200 soldiers.

Recruits often come with handwritten paperwork vouching for them, and officials say that is sometimes enough to accept them.

While the NATO training mission is putting new recruits through a biometric database, that is only useful for those already in the system. There is very little documentation on most rural people in Afghanistan.

NATO training officials have no data regarding how many times army or police training centers have been infiltrated by insurgents, according to spokesman Maj. Kevin Heineman. There are no systems in place that allow for thorough cross-checking of recruits between recruitment centers and NATO or the interior ministry, he said.

Earlier this year, a recruit who was believed to be a Pakistani Taliban member was caught on the Adraskan base, after he was overheard speaking Pakistani Urdu, according to Afghan Lt. Col. Shafiqullah Tehari, head of training at the base.

He told the authorities that he was speaking with relatives in Pakistan, Tehari said. But his documents were questionable and he was sent to Kabul, where, according to one report, he was arrested and sentenced to prison.

“It’s hard to prevent this,” Michael O’Hanlon, an Afghan expert at the Brookings Institution in Washington, said by email Monday. “One can improve vetting, but how can you be sure about the loyalties, or lack thereof, of people you don’t know. It’s a quandary.”

Stars and Stripes correspondent Geoff Ziezulewicz reported from Herat Province and correspondent Dianna Cahn reported from Kabul.

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