Money from a multibillion-dollar U.S. transportation contract in Afghanistan has made its way into the hands of the Taliban, The Washington Post reported Sunday, citing an unreleased investigation by the U.S. military.

The Post said the probe found evidence of "involvement in a criminal enterprise or support of the enemy" by four of the eight companies contracted by the U.S. to provide transportation services in Afghanistan. It also said contractors were involved in money laundering, profiteering, kickbacks and fraudulent paperwork.

The newspaper said all eight of the companies are still working on the $2.16 billion contract, designed in part to promote Afghan businesses.

In one case, intelligence officials traced $3.3 million through a series of subcontractors, an Afghan National Police commander, and eventually to insurgents in the form of cash, weapons and explosives, the Post reported.

A U.S. defense official told the Post that the transport system will be radically revised after the trucking contract expires in September.