KABUL, Afghanistan — An Afghan employee of the U.S. government opened fire inside a CIA office in Kabul on Sunday evening, killing an American and injuring a second, U.S. and Afghan officials said, in the second major breach of embassy security in two weeks.

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The killing adds to a sense of insecurity already heightened by a 20 hour-siege of the embassy district in mid-September, and the assassination a week later of the government's peace envoy, former President Burhanuddin Rabbani.

The CIA compound is one of the most heavily guarded in Kabul, and has been off-limits for almost a decade, since shortly after the Taliban's fall from power in 2001.

It also lies at the heart of the capital's heavily-guarded military, political and diplomatic district, a virtual "green zone" that is almost impossible for ordinary Afghans to enter.

It was not clear if the U.S. citizens were victims of a rogue employee who had been won over to the insurgent cause, or just the escalation of an argument in a city were tensions are high and many people carry guns. There are precedents for both.

U.S. military officials did tell NBC News that so far there appears to be no connection between the shooter and the Haqqani or Taliban terror networks.

The "lone attacker" was killed, and the injured U.S. citizen was taken to a military hospital, U.S. embassy spokesman Gavin Sundwall said on Monday.

"There was a shooting incident at an annex of the U.S. embassy in Kabul last night involving an Afghan employee who was killed. The motivation for the attack is still under investigation at this time," Sundwall said.

The shooting follows a string of attacks by Afghan security forces against their NATO-led mentors carried out either by "rogue" soldiers and police or by insurgents who have infiltrated security forces.

The Taliban could not immediately be reached for comment.

'Serious trust' issues
Waheed Mujhda of the Afghan Analytical and Advisory Center in Kabul, said the incident characterized the level of mistrust between the United States and its Afghan allies.

"It's not the first time this has happened and it won't be the last, he said.

"This is a big security concern for the Americans and it shows they can't fully trust their Afghan staff. But the Americans never want to accept that there are serious trust and cooperation issues and they have encountered that in their security operations with Afghan forces."

The shooting happened at the Ariana hotel, just a few blocks away from the Presidential Palace and the U.S. embassy, and used by the Central Intelligence Agency as a Kabul base.

Kabul Police Chief Ayub Salangi said there had been an exchange of fire at the hotel, which he described as an "office" for the CIA, but declined further comment on what happened in an area where access is restricted even for Afghan forces.

The shooting came the same month that insurgents took over an unfinished high-rise near the city's heavily guarded military, political and diplomatic heart and showered rockets down on the U.S. embassy and NATO headquarters.

That attack lasted 20 hours, and the U.S. has blamed it on the Haqqani network of militants, who were long based in Pakistan's lawless frontier regions although they now say they have moved back into Afghanistan.

Washington accused Pakistan's spy service of offering them support.

But senior Pakistani officials have lashed out against the allegations, accusing the U.S. of trying to make Pakistan a scapegoat for its troubled war in Afghanistan.

The U.S. has given Pakistan billions of dollars in military and economic aid, but the relationship has been riven by mistrust.

'Big loss'
Sunday's assault also follows closely on last week's assassination of former Afghan President Rabbani, who was leading a government effort to broker peace with the Taliban. He was killed when an insurgent who had claimed to be a peace emissary exploded a bomb hidden in his turban upon meeting Rabbani.

President Hamid Karzai called Rabbani's death a "big loss" and said greater security measures should be taken to protect top Afghan figures, including religious clerics and tribal leaders. Intelligence officials have said one person has been arrested in connection with the assassination and that authorities were close to uncovering the details of the killing.

On Monday, the National Directorate of Security said an Afghan suspect had revealed the attack on Rabbani was plotted outside the country.

Zeya, deputy head of the NDS, who like many Afghans goes by only one name, suggested the Quetta Shura, the Taliban's leadership council, may have played a role in Rabbani's killing and said the NDS would recommend to President Hamid Karzai that he push for the investigation to be taken beyond Afghanistan's borders.

NATO bases and embassies have ramped up security following a number of attacks over the past year by Afghan security forces against their counterparts. Since March 2009, the coalition has recorded at least 20 incidents where a member of the Afghan security forces or someone wearing a uniform used by them killed coalition forces. Thirty-six coalition troops have died. It is not known how many of the 282,000 members of the Afghan security forces were killed.

In December 2009, an al-Qaida double agent blew himself up at a CIA base in eastern Khost province, killing seven CIA employees. The attacker, a Jordanian man named Humam al-Balawi, had been brought into the base because he had claimed to be able to reach high-level al-Qaida leaders.

In the south, meanwhile, a NATO service member was killed in a bomb attack Monday, making a total of 38 international troopers killed so far this month.