KABUL, Afghanistan >> In the early afternoon of Nov. 29 at an outpost high in the mountains along the Pakistan border, an Afghan border policeman well thought of by his superiors suddenly opened fire on American soldiers, killing six.

The Taliban rushed to take responsibility for the attack, claiming the gunman was a sleeper agent planted to kill NATO soldiers. The Afghan police and military scrambled to find ways to root out insurgents lurking within their ranks. Screening of new recruits and soldiers intensified.

But when a joint inquiry was completed sometime later, Afghan and American investigators concluded that the gunman, a man named Ezzatullah from a small village in Nangarhar province, where the attack occurred, was not a sleeper agent at all but a good soldier overcome by personal stress, including his father’s insistence that he accept a marriage contract with a young girl.

Fears over Taliban infiltration in the Afghan national security forces arose again this week after insurgents dressed in Afghan military uniforms attacked three heavily secured government locations. The latest attack came Monday on the Defense Ministry’s headquarters in downtown Kabul and killed two Afghan soldiers.

The attacks have stoked concerns among Afghan officials, who are uneasy about their own safety and the fate of a country whose military and police forces they worry might be infused with enemy insurgents. Some in the Senate on Wednesday called for the resignation of the defense minister, Abdul Rahim Wardak, calling him incapable of defending his own ministry, much less the country.

But preliminary inquiries show that the perpetrators in at least two of the attacks were not serving members of the Afghan military or police, according to two senior NATO intelligence officers and a senior NATO adviser briefed on the investigations. They are also doubtful that the person in the remaining attack was a serving member, though the investigation is continuing.

In fact, intelligence officials have gathered no evidence to suggest that infiltration is widespread, as the Taliban claims, according to the officers, who spoke on condition of anonymity because of the nature of their positions advising Afghan forces. Nonetheless, officials know that Taliban claims of infiltration breed distrust, and are hard to disprove.

“Their goal is to separate the coalition from the Afghan National Army, and this is a great tool for them, whether they’ve done it or not,” one of the officers said.

Infiltration or not, the recent attacks have exposed other security concerns, including the rigor of identification checks and body searches at checkpoints and entryways, and the easy availability of official-looking uniforms and military equipment in shops and bazaars throughout Kabul and the provinces. Investigators are also examining whether any guards took bribes to let the insurgents through.

“At least two of them are quite clearly guys who had gotten a uniform and had been helped,” the NATO security adviser said. “And that’s really what we’re concerned about is, is the enemy able to penetrate through this vetting system, or are they actually able to co-opt or utilize uniforms and other equipment that they’re able to gather out on the street and exploit weaknesses in the physical security of these places.”

Concerns over sleeper agents still run high among NATO and Afghan officials. After the November attack in Nangarhar, coalition forces sharply escalated training of Afghan counterintelligence agents. It is their job to identify possible insurgents among Afghan forces and to look for signs of service members who, acting either out of financial or personal stress or because of threats to their families, might fall under Taliban influence. Nearly 200 agents are in the field now, and that number is expected to more than double by year’s end.

Since September 2009, when NATO forces began intensifying efforts to build the national security forces, all recruits have been required to go through an eight-step screening process. As part of it, recruits must undergo criminal background checks and drug tests, and must present two letters from village elders vouching for their character.

Biometric data is also collected on each recruit, including iris scans and fingerprints. That data is compared against watch lists of suspected insurgents whose fingerprints have been found on improvised explosive devices and other ordnance. But a significant percentage of the 159,000 soldiers and 125,000 police officers now serving have never been biometrically scanned, because they joined before such screening began. Officials could not provide the exact percentage Wednesday.

Last month the army and police began collecting data on all their members, even those recruited after screening began, a process that is expected to take about eight months.

Since March 2009, NATO has counted 16 cases in which Afghan service members have turned their weapons on coalition soldiers. Termed “green on blue” attacks, they have led to 38 coalition deaths. But in about half of those, including the November attack, investigators determined that the attackers did not act on behalf of insurgents, as often claimed by the Taliban, but for a combination of other reasons, including battlefield stress and personal animosity against coalition soldiers.

On April 4, for example, a border police officer killed two American soldiers in the northern province of Faryab. The investigation revealed that the officer was killed in reaction to the burning of a Quran at a Florida church run by Terry Jones. And on Feb. 18, an Afghan soldier opened fire on German soldiers in Baghlan province, killing two of them and wounding eight others. Again, the investigation concluded that built-up anger between the soldier and the Germans had led to the shooting.

“He was a quality soldier who for whatever reason had arguments with these German soldiers and then one day just shot them,” the adviser said.

There seems to be little question that insurgents were responsible for the latest attacks, which the Taliban called the beginning of their spring offensive. On Saturday, a suicide bomber wearing an Afghan army uniform killed five NATO service members, an American contractor and four Afghan security force members in Laghman province.

A day earlier, a suicide bomber wearing a police uniform killed the widely respected police chief and two police officers in Kandahar. In those and in the case of Monday’s attack on the Ministry of Defense, the infiltrators were able to penetrate heavily secured buildings.

Gen. Zahir Azimi, a spokesman for the Ministry of Defense, said Monday’s attacker, who was killed before he could detonate his suicide vest, was clearly not a member of the military. Investigators for now believe that he entered in a vehicle with Defense Ministry tags, and they are examining whether he had help from workers inside.

“The initial investigation shows gaps in security,” Azimi said at a news conference Wednesday. “Now we’re working to fill those gaps.”

© 2011 The New York Times Company