Story highlights

NATO, Afghanistan investigators are probing Wardak province incident

The latest attack could be the first time an Afghan army unit attacked U.S. troops

General said "insider attacks" could emerge as insurgents' "signature attack"

The military is working to improve vetting of Afghan military recruits

(CNN) —  

A deadly assault on American forces in eastern Afghanistan over the weekend stokes fears of a disturbing new form of “insider attack” – an assault on coalition forces by an Afghan military unit rather than a lone attacker.

U.S. forces apparently took fire on Saturday from several Afghan troops shooting at them from several directions, according to a U.S. military official familiar with initial results of the investigation.

NATO and Afghan officials investigating the Wardak province assault are expected to make their findings public soon, maybe as early as Wednesday, the official said.

The official declined to be identified because the findings have not yet been made public.

Marine killed in Afghanistan predicted insider attack that lead to his death

The Wardak assault comes amid as concern reached crisis proportions over so-called “insider” or “green on blue” attacks. They are the names for strikes by Afghans soldiers or police – or militants wearing their uniforms – on U.S. and allied forces.

If the initial suspicions about Saturday’s attack are borne out, it would be the first time U.S. troops have come under attack from an Afghan unit rather than lone attackers.

The Saturday incident apparently started when an Afghan soldier fired his weapon, suddenly killing the U.S. soldier and civilian contractor.

U.S. and Afghan troops had a “short conversation” before the firing began. The U.S. official said that the Afghan soldier was known to both the Americans and the Afghans; so, the initial firing is being classified as an insider attack.

American troops fired back killing the Afghan and a man standing next to him.

At that point, the U.S. troops report they came under fire from multiple persons and multiple directions.

NATO has said it has reports that some of the firing was possibly conducted by nearby insurgents. The U.S. official said it strongly appears Afghan forces also fired on the Americans from multiple points.

The initial conclusion from NATO and Afghan officials is that U.S. forces may indeed have also come under insurgent fire at the same time.

But, there are still strong indications multiple Afghan troops fired on the Americans. At this point, it’s not been made public how much of the fire was deliberate, or if there was some battlefield confusion during the event.

While NATO forces want to put the findings out as soon as the investigation is done, it still could run into problems. One reason could be that because this is a joint investigation with the Afghans, the government of President Hamid Karzai must agree to findings which could paint Afghan forces in a poor light.

NATO is already aware this incident is garnering widespread attention in military circles, because of the potential that multiple Afghan troops attacked the Americans. “We know there is a lot of interest,” the official said. He emphasized the incident involved a lot of confusion, but NATO believes it will be able to determine what happened.

ISAF: Insurgents in deadly attack in Afghanistan wore U.S. Army uniforms

Insider attacks have been on the rise for months. The senior combat leader in Afghanistan, Gen. John Allen, told CBS’ “60 Minutes” Sunday night that he is “mad as hell” about them.

“We’re willing to sacrifice a lot for this campaign, but we’re not willing to be murdered for it,” he said.

The attacks have occurred during a crucial period in the Afghan war – the training of a fledgling Afghan army. Coalition forces hope to get Afghan security forces ready to take charge of security after NATO-led troops withdraw from Afghanistan at the end of 2014.

Measures have been devised to curb and thwart the insider attacks.

Soldiers are required to have a loaded weapon within reach at all times.

The military is improving its vetting of new recruits and working to address cross-cultural issues that lead to tensions. Training ended for hundreds of Afghan soldiers while their backgrounds are checked further for insurgent links.

NATO’s International Security Assistance Force said a regional commander now must give the OK for a joint U.S.-Afghan action, a move that has limited operations. Some troops have been assigned to be “guardian angels,” soldiers in units who keep an eye on the Afghan forces.

The military attributes some of the attacks to Taliban infiltration and also to cross-cultural misunderstandings that escalated into violence.

Secretary of Defense Leon Panetta recently called the insider attack problem in Afghanistan “a last-gasp effort to be able to not only target our forces, but to try to create chaos, because they’ve been unable to regain any of the territory that they have lost.”

One of Washington’s top military analysts pushed back at the “last gasp” remark.

“Quite frankly, I think that most intelligent people and military people would privately think that Secretary Panetta’s comments are absurd, perhaps harmful. Because they just can’t be taken seriously,” said Anthony Cordesman, a senior analyst at CSIS, a major Washington think tank, and a recipient of Department of Defense Distinguished Civilian Service Award for his work at the Pentagon before joining CSIS.

“They are not at the last gasp, all they really have to do is – at this point – out-wait us, constantly put pressure on areas that give them political visibility. They don’t have to defeat ISAF, it’s leaving.”

On “60 Minutes,” Gen. Allen said the insider attacks have emerged because the “enemy” has seized on a “vulnerability.”

“In Iraq, the signature weapon system that we hadn’t seen before was the IED,” Allen said, referring to improvised explosive devices “We had to adjust to that. Here, I think the signature attack that we’re beginning to see is going to be the insider attack.”

Opinion: Green-on-blue attacks show there’s no easy way out of Afghanistan

CNN’s Larry Shaughnessy and Joe Sterling contributed to this report