WASHINGTON — 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.