A Senate report has found evidence that many Afghan security personnel paid with US taxpayers' money to guard American bases are hand in glove with the Taliban insurgents hell-bent on killing coalition troops.
One disturbing case uncovered by the Armed Services Committee in the western Afghan province of Herat illustrates the deadly double game played by some of these hired guns.
Two Afghan warlords involved were nicknamed by their British employers as Mr White and Mr Pink, after gangsters in the gory Hollywood movie Reservoir Dogs.
The pair were appointed in June 2007 by the defence contractor ArmorGroup to provide security at Shindand airbase.
The Senate report links the warlords, and their successors, to "murder, kidnapping, bribery and anti-coalition activities" during their 18 months on the payroll.
In December 2007, Mr White was ambushed and shot dead in a bazaar just outside the airbase - by Mr Pink and his men.
An American contactor said the shooting amounted to a mafia-style hit by Mr Pink to rub out a business rival.
Afterwards, it emerged that Mr Pink was holed up in a nearby village with a number of Taliban fighters.
Despite this, ArmorGroup continued to employ Mr Pink's men for more than a month, the Senate committee found.
The firm finally sacked the men after suspicions were aroused that they were providing intelligence about the Americans to Mr Pink.
Mr White II
ArmorGroup, meanwhile, turned to the late Mr White's brother, who became known as Mr White II, to fill the vacancy.
Mr White II was the uncle of a high-value Taliban commander, Mullah Sadeq, who was directing roadside bombings in Herat and the neighbouring province of Farah.
In summer of 2008, an ArmorGroup mine-clearing company, AGMA, was awarded a United Nations contract to clear explosives from Herat.
The firm hired Mr White II to provide security, paying him thousands of dollars a month.
It later emerged that he was using some of this money to fund Taliban operations.
In August 2008, US forces received intelligence that Mullah Sadeq and other insurgents were at a compound in the village of Azizabad, not far from Shindand airbase.
The meeting, notes the Senate report, was being held in the home of Mr White II, who was also there, along with a number of colleagues.
US forces raided the property and a ferocious firefight ensued, resulting in a number of casualties, including civilians.
Mr White II and seven guards employed by ArmorGroup were among those killed.
Having learned its lesson, ArmorGroup decided that Mr White II's men could no longer be trusted, and sacked them.
But AGMA continued to employ them, even hiring brothers of the men killed in the Azizabad raid.
Mr White III
The firm also reached agreement with Mr White II's brother to take over as the company's security provider.
AGMA assessed that Mr White III, as he would be known, "was a man we could do business with".
But it was not long before Mr White III's loyalties were being questioned, too.
As roadside bombings in the area began to increase, AGMA began to view Mr White III and his men as a potential threat.
But the firm still kept them on the payroll until its UN contract wound up in December 2008.
ArmorGroup told the BBC it was unable to comment on the detail of the report, but it issued the following statement: "ArmorGroup engaged workers from two local villages as stated by the Senate Report - but did so only upon the recommendation and encouragement of US special forces.
"ArmorGroup personnel remained in close contact with US special forces personnel to ensure that the company was constantly acting in harmony with, and in support of, US military interests and desires."
'Hostile foreign government'
The Senate report found that other US defence contractors used double-dealing local muscle.
EOD Technology hired an ex-police officer to provide security for a police training centre in Herat in 2008.
US military intelligence discovered that the man worked for a "hostile foreign government".
The committee also found how little vetting was carried out on Afghan security firms.
An audit in September 2008 at a base in the eastern province of Nangarhar found one contractor could not even produce a roster of its own employees.
And in March the following year, all the Afghan guards at a base in the neighbouring province of Kunar were sacked after it emerged their boss was linked to a terrorist organisation, says the Senate report.
Other private security personnel, meanwhile, were found to have barely any experience or weapons training.
One guard said he had not fired a gun since the 1980s.
As of May this year, there were 26,000 private security personnel in Afghanistan, according to the Senate Armed Services Committee. The vast bulk of the private guards are Afghans.
Nine out of 10 of them were employed as contractors or subcontractors to the US government.
President Hamid Karzai has declared that private security firms should be phased out by the end of this year, with the aim of integrating them with the Afghan police and army.
But the committee found that members of the security forces were quitting for better-paid jobs in private security.
Afghan guards working for ArmorGroup in Herat were paid $275 (£170) a month, not including expenses, says the report.
This was four times more than the $70 monthly pay of an Afghan police patrolman, as of February 2008.
The Senate committee noted that "the safety of our troops and the success of our mission require immediate and aggressive steps" to remedy the problem posed by Afghan private security firms.
With US President Barack Obama aiming to begin a US withdrawal from Afghanistan by July 2011, the report indicates much work remains to ensure Afghan security forces are ready to take over.