“They’ve got to screen everybody who’s going to be working directly with the [brigade],” said an Army officer who was involved in preparing bases for the new adviser teams earlier this year and who, like others contacted for this story, agreed to speak on the condition of anonymity. “That means screening basically the whole damn Afghan National Army, and we’re way behind the power curve on that.”

The vetting effort is meant to protect members of the brigade from “insider attacks” by the Afghan troops they are working with, which for years have plagued U.S. and NATO efforts to help establish a viable Afghan force to secure the war-torn nation.

“We want to be deliberate and diligent about the way we’re partnering to ensure that we’re being safe,” a spokesman for the U.S.-led military headquarters in Kabul, Lt. Col. Martin O’Donnell, said. “Systems like the screening ensure that.” He added that “not everyone in a unit needs to be vetted for advising to begin.”

The adviser brigade, based at Fort Benning, Ga., consists of about 1,000 advisers and security personnel. Its deployment to Afghanistan was accelerated at the request of Defense Secretary Jim Mattis to get it in the fight in time for the warm-weather fighting season in Afghanistan. It accounts for about a quarter of the 4,000 additional American troops dispatched to the country since last summer — reinforcements that have brought the total U.S. contingent to about 15,000.

The top commander in Afghanistan, Army Gen. John Nicholson, has stressed the importance of pushing adviser teams down to the lower levels of the Afghan army — specifically to battalions, or kandaks, which are units of a few hundred Afghan soldiers.

U.S. Special Forces have advised some elite Afghan commando units at that level for years, but the regular Afghan army has not had advisers with its battalions since 2014.

The new advisory brigade is “made up of volunteers who are then specially trained in a range of skills to provide combat advising at the tactical level,” Nicholson told reporters late last year, as the brigade was preparing to deploy. “So they’ll go down to the kandak level, the battalion level, which is really where we have operated successfully for the last couple of years with our Special Forces advisers.”

But a desire to prevent casualties among the advisers from “insider attacks” has driven the U.S. headquarters in Afghanistan to institute a massive screening process before letting the adviser teams move out with that part of their mission, the officials said.

In 2012, such attacks accounted for a full quarter of U.S. and NATO casualties, and the Taliban touted their effectiveness.

The Afghan army has suffered massive casualties since the U.S. and NATO military presence was significantly reduced in 2014. Last year alone, more Afghan soldiers and police were killed than the total American death toll in the 17-year war.

Desertion is rampant. A recent U.S. government assessment found that the supposedly 334,000-strong army actually has fewer than 300,000 troops. Many recruits are illiterate and from the same rural backgrounds as the Taliban, which has taken advantage of that fact to seed the force with infiltrators.

Nevertheless, the extensive U.S. effort to vet its allies is controversial, sparking criticism that the American command is being overly cautious.

Michael Waltz, a retired Special Forces lieutenant colonel who has advised Afghan commandos, said the large-scale screening process suggests U.S. commanders are too risk-averse, fearful of taking too many American casualties.

“You cannot have a zero-defect, zero-casualty mentality when it comes to an advisory effort in one of the most difficult and complex parts of the world,” Waltz said. “You have 17 years of accumulated rules and regulations and policies, each one of which made sense at the time they were enacted, but if you step back and look at them in total, we’re tying our hands.”

Such an extensive vetting program is “a bureaucratic solution that fundamentally undercuts the purpose of these adviser teams and demonstrates a lack of commitment to the mission,” added Jason Dempsey, a former Army officer who served as a combat adviser in Afghanistan and is now with the Center for a New American Security.

The Army advisory brigade itself maintains that it is already contributing significantly to the mission, despite the lack of battalion-level advising.

“In general, we’re being used as designed,” said a spokesman for the brigade, Maj. Matt Fontaine. “We’ve only been on the ground, the full brigade, for less than a month. We feel like we’re moving at an appropriate speed.”

But he also acknowledged that the adviser teams have yet to pair up with Afghan battalions as Nicholson said they would.

So far they are working only with the larger Afghan headquarters units that already had American advisers attached. “We’re setting the conditions to go down to the kandaks,” Fontaine said. “We want to keep ourselves secure and safe through this process.”

O’Donnell, the spokesman for the U.S.-led headquarters in Kabul, appeared to contradict that, saying that the new brigade “has advised from the corps to the kandak level” although most of its advising has been at the higher levels.

He also explained that only so many new advisory teams can fan out at a time due to security reasons. Before any of the teams actually move in with Afghan battalions at their far-flung bases, they have to make preliminary visits to assess security.

At the same time, an array of artillery, drones, jets, helicopters and security personnel have to be lined up before the visits.

The Air Force recently deployed extra Reaper drones and A-10 attack jets to Afghanistan to support the Army advisory brigade, but “those resources are not unlimited,” Fontaine said. “We can’t send 12 or 14 teams out all at the same time if they’re all competing for the same” aircraft and equipment.

Counterintelligence personnel — typically civilian contractors — come along on the visits to conduct biometric screening of the Afghan troops the advisers will be working with, according to a contractor involved in the effort earlier this year.

The screening involves the collection of retinal scans, fingerprints and DNA swabs, the contractor said, which are then checked against a database for red flags like connections to detained Taliban fighters.

Such screening is routine for the smaller Afghan commando force that works with Army Special Forces. But the military and the company responsible for a large part of the screening, CACI International, have struggled to scale the process up for the much larger regular army, which has many more bases, the contractor said.

The counterintelligence contractors have yet to visit many of the bases where the new U.S. advisory teams will need to live to carry out their task, the contractor said.

CACI declined to discuss its vetting work in Afghanistan, and the top U.S. headquarters in Afghanistan did not respond to multiple requests for comment.

Fontaine, the spokesman for the adviser brigade, also alluded to the challenge of vetting so many troops.

“The screening is a process that’s been ongoing for some time in Afghanistan, but like everything else associated with the [brigade], we have to take it to the next level,” Fontaine said.

The plan, Fontaine acknowledged, is for all Afghan troops who will come in contact with the advisers to undergo screening — a massive task.

“Depending on the situation, we may be able to take a little bit of risk” and allow teams to work at bases where screening has not taken place, Fontaine said, but “we’re very cautious on how we do that.”

“It’s proceeding,” he added. “We’ve seen people enter and go through and complete the screening process in the weeks we’ve been here,” he said, adding wryly, “it’s working as smoothly as any other government process.”

O’Donnell, the spokesman for the U.S.-led headquarters in Kabul, also warned against overplaying the importance of embedding U.S. advisers at lower levels of the Afghan army — despite emphasis from top commanders in recent months.

“We’ll partner at whatever level has the most return on investment for the Afghans,” he said. “We have the authority to go to the kandak level if we need to, and in some cases we’re doing that, but in others we’ve found that we’re more effective” at higher levels.