The female Afghan police officer who shot dead a NATO adviser in Kabul is an Iranian national who wanted to kill senior security members, officials said Tuesday.
On Monday an Afghan police officer opened fire on a NATO civilian adviser inside police headquarters, killing the adviser who sources said was American.
It was the first "insider" attack by a woman in a series of such attacks, which have seriously undermined trust between NATO forces and their Afghan allies in the fight against hardline Islamist Taliban insurgents.
The officer, named only as Nargis, was later arrested and prosecuted by the police.
"Our investigation shows that Nargis is an Iranian national. After her marriage with an Afghan she managed to obtain an Afghan ID illegally and joined the police," interior ministry spokesman Sediq Seddiqi told reporters in Kabul.
"Our investigation also shows she was suffering from a psychological instability and our understanding from the past 24-hours investigation is that she is not associated with any armed opposition groups," he said.
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Mohammad Zaher, head of the criminal investigation department of Kabul, said Nargis had married an Afghan man 10 years ago and joined the police five years later.
"In her confession she has said that she was tired of life and wanted to kill either the governor, the CID chief or the police chief," Zaher said.
"But after she failed to get into the police HQ, she shot a foreign national she saw near the canteen," he said.
NATO is aiming to train 350,000 Afghan soldiers and police by the end of 2014 as it transfers all security responsibilities to President Hamid Karzai's local forces.
The Afghan conflict has seen a surge in insider attacks this year, with more than 50 ISAF soldiers killed by their colleagues in the Afghan army or police, though most have taken place on military bases and not in the capital.
US special forces suspended training for around 1,000 Afghan Local Police recruits in September to re-investigate current members for possible links to the Taliban, after the rise in insider attacks.
Training for the national police was not affected.
NATO forces will complete their withdrawal from the war-torn country in 2014, after a war with the Taliban militants that began with the US-led invasion in 2001.