WASHINGTON (AP) — Criminal investigators are examining allegations that Afghan security firms have been extorting as much as $4 million a week from contractors paid with U.S. tax dollars and then funneling the spoils to warlords and the Taliban.

If the allegations are true, the U.S. would be unintentionally financing the enemy and undermining international efforts to stabilize the country.

The payments reportedly end up in insurgent hands through a $2.1 billion Pentagon contract to transport food, water, fuel and ammunition to American troops stationed at bases across Afghanistan. To ensure safe passage through dangerous areas, the trucking companies make payments to local security firms with ties to the Taliban or warlords who control the roads. If the payments aren't made, the convoys will be attacked, according to a U.S. military document detailing the allegations being examined by investigators.

The document says the companies hired under the Afghan Host Nation Trucking contract may be paying between $2 million and $4 million a week to insurgent groups.

Chris Grey, a spokesman for the Army Criminal Investigation Command at Fort Belvoir, Va., confirmed Monday that the inquiry is under way. But he said he would not provide details in order "to protect the integrity of the ongoing case."

One of the firms under scrutiny is Watan Risk Management, one of the largest security providers in Afghanistan and owned by two cousins of Afghan President Hamid Karzai.

Watan representatives allegedly negotiate or dictate the price for security in a given area, according to the document, and also issue warnings to trucking companies that are late in paying or refuse to do so.

A woman who answered the telephone at Watan's office in Kabul said the company would have no comment and hung up.

A new report from a congressional oversight subcommittee that conducted its own investigation of the trucking contract gives weight to the claims of corruption. Its report, released Monday, says the trucking contractors pay tens of millions of dollars annually to local warlords across Afghanistan in exchange for guarding their supply convoys.

"Although the warlords do provide guards and coordinate security, the contractors have little choice but to use them in what amounts to a vast protection racket," says the report by the Democratic staff of the House Oversight and Government Reform national security subcommittee. "The consequences are clear: Trucking companies that pay the highway warlords for security are provided protection; trucking companies that do not pay believe they are more likely to find themselves under attack. As a result, almost everyone pays."

The report also said the insurgents aren't the only beneficiaries. One security company told the subcommittee that it had to pay $1,000 to $10,000 in monthly bribes to nearly every Afghan governor, police chief and local military unit whose territory the company passed through.

The subcommittee's report cites Watan Risk Management's relationship with a warlord, Commander Ruhullah, in the southern city of Kandahar. Ruhullah commands a small army of 600 armed guards that dominate the security business on Highway 1, the main artery between Kabul and Kandahar.

Ruhullah told the congressional investigators that he guards about 3,500 U.S. supply trucks every month and charges about $1,500 per truck. That works out to $5.2 million every four weeks.

"While a small handful of security companies apparently do operate convoy security missions on this route without paying Commander Ruhullah, they do so at their peril," the report says.

Ruhullah and Watan's owners denied paying the Taliban, which they describe as a decentralized organization made up of different tribes and groups. Ruhullah said the frequent firefights his guards have with the Taliban are evidence that he does not pay them, according to the subcommittee's report.

The subcommittee, chaired by Rep. John Tierney, D-Mass., is holding a hearing Tuesday on its findings.

Rep. Darrell Issa, R-Calif., said Monday that Tierney's investigation may have jeopardized the military's inquiry. Issa, the top Republican on the House Oversight and Government Reform Committee, said several witnesses who spoke to congressional investigators are now not cooperating with the Pentagon's criminal probe. He did not explain why.

Issa also criticized Tierney's investigators for a lack of thoroughness by failing to record witness statements. He said at least 30 witnesses were questioned by the subcommittee's staff and the only record of the interviews were their handwritten notes.

A spokeswoman for Tierney declined to respond to Issa's criticisms, saying the subcommittee's report "speaks for itself."

The Host Nation Trucking contract is a critical component of the effort to keep more than 200 U.S. military combat outposts throughout the country stocked. Supplies are typically shipped through Pakistan to Bagram Airfield, the U.S. military's main hub in Afghanistan, and then on to the outlying bases.

The long, rugged supply lines to landlocked Afghanistan through Pakistan's port city of Karachi offer numerous opportunities for fraud and corruption that pad the insurgent's accounts.