By Hamid Shalizi

KABUL (Reuters) - An Iranian-Afghan policewoman who killed a U.S. contractor at the police headquarters in Kabul may have been motivated by a personal grudge, said security officials, who were also probing possible Taliban or al Qaeda involvement.

The officials said the woman named as Narges seemed wracked with remorse over the shooting. They said she held an Iranian passport but offered no evidence that Iran may have orchestrated the attack.

She arrived at police headquarters on Monday morning and headed to a bathroom where she loaded a pistol and hid it under her long scarf, they said. She then approached the American police trainer as he was walking to a canteen and shot him in the ribs.

"After she shot the American, she pointed her pistol to other policemen who rushed to arrest her. But her weapon jammed," one top police official told Reuters. "Her prime target could have been senior officials in the compound."

It appeared to be the first time that a female member of Afghanistan's security forces had carried out such an attack.

At least 52 members of the NATO-led International Security Assistance Force (ISAF) have been killed this year by Afghans wearing police or army uniforms, in so-called insider attacks.

Also known as green-on-blue attacks, the incidents have undermined trust between coalition and Afghan forces who are under mounting pressure to contain the Taliban insurgency before most NATO combat troops withdraw by the end of 2014.

"This was a very organized shooting," said the top police official. "There must have been bigger hands involved... At this stage, we can only say that she could have been brainwashed either by the Taliban or al Qaeda."

Virginia-based DynCorp International described the killing of its employee, police mentor Joseph Griffin, 49, as a tragedy.

"The loss of any team member is tragic but to have this happen over the holidays makes it seem all the more unfair," the company said on its website.

IRANIAN PASSPORT Continued...