KANDAHAR, Afghanistan - The two Land Rovers and another four-by-four rumbled through a mountain pass between two provinces in central Afghanistan. The 40 or so passengers in the convoy were heading south to Kandahar province to find work.

They came from Dai Kondi, an enclave of the country's Hazara ethnic group that's known for its almonds.

But to the NATO helicopters hovering overhead, the three vehicles were supposed to be crawling with insurgents, reportedly fleeing a major military offensive hundreds of kilometres away.

At least 21 of the passengers died when the choppers opened fire. A dozen or more were wounded in a hail of helicopter fire.

Two days after Afghanistan's deadliest attack on civilians in six months, many questions remain unanswered. Perhaps the two most pressing are: Who called in the air strike? And on what grounds?

Dual investigations by NATO and the Afghan government are underway to answer those questions. But the cabinet of Afghan President Hamid Karzai has already made it clear the attack was, in a word, "unjustifiable."

The New York Times, quoting an Afghan army commander in Uruzgan province, reported Tuesday that U.S. Special Forces helicopters were hunting for militants who had escaped from a NATO offensive in Helmand province some 240 kilometres away.

That report couldn't be independently verified. A spokesman for the Interior Ministry could not provide any more information about the attack.

But more details about the convoy emerged from the governor of Uruzgan, Assadullah Hamdam, who said he spoke to some of the wounded passengers.

"The people mentioned to me, the wounded people, they wanted to travel to Kandahar from Dai Kondi ... for work," Hamdam told The Canadian Press.

Karzai has been the face of Afghans' anger over the mounting civilian death toll.

The president stood in parliament barely a day before the Uruzgan attack and urged NATO to do more to protect civilians as it tries to rout a recalcitrant insurgency.

The military alliance has lately limited air strikes and tightened its rules of engagement on the battlefield to try to protect the Afghan people.

So worried are coalition forces about losing public support for the military mission that the top commander of U.S. and NATO forces here took his apology directly to the Afghan people.

"We are extremely saddened by this tragic loss of innocent lives. I have made it clear to our forces that we are here to protect the Afghan people," a stern-looking Gen. Stanley McChrystal said in a video broadcast on national television Tuesday in Pashto and Dari.

"I pledge to strengthen our efforts to regain your trust to build a brighter future for all Afghans."

But to some McChrystal's apology rang hollow.

"All the time they shoot civilians and say sorry, which makes no sense to me," said Amanullah Hotaki, the head of Uruzgan's provincial council.

Winning hearts and minds -- a euphemism for local support popularized during the Vietnam war and later applied to campaigns in Iraq and Afghanistan -- is one of the main objectives of Afghan war effort.

But support ebbs every time a civilian dies from NATO fire, every time a home is demolished under a tank's treads or obliterated by an errant rocket.

Part of the problem is an insurgency that can vanish like spectres into the villages and towns of Afghanistan.

The militants have no qualms about using civilians as human shields, as they've done throughout the ongoing military offensive in Helmand province.

Afghan and foreign forces are in their 10th day of fighting insurgents in the town of Marjah and the district of Nad Ali as part of Operation Moshtarak.

That assault -- the biggest since the fall of the Taliban in 2001 -- has claimed the lives of at least 16 civilians. Most of them died after a pair of errant NATO rockets smashed into a house in Marjah.

And an air strike in northern Kunduz province Thursday that was meant to wipe out a group of insurgents missed and killed seven Afghan police officers.