By NATHAN HODGE

WASHINGTON—A yearlong investigation by a Senate panel has found evidence that the mostly Afghan force of private security guards the U.S. military depends on to protect supply convoys and bases in Afghanistan is rife with criminals, drug users and insurgents.

The Senate Armed Services Committee inquiry, based on interviews with dozens of military commanders and contractors and a review of over 125 Pentagon security contracts, found evidence of “untrained guards, insufficient and unserviceable weapons, unmanned posts” and other failings that put U.S. troops at risk.

More alarming, the report alleges that some local warlords who have emerged as key labor brokers for private security firms are also Taliban agents.

Sen. Carl Levin (D., Mich.), the chairman of the committee, said failures to adequately vet private security contractors in Afghanistan poses “grave risks” to U.S. and allied troops. The overall lack of proper contractor supervision, he added, poses a fundamental threat to the U.S. mission.

Afghan President Hamid Karzai has ordered that all security firms in Afghanistan be dissolved by the end of the year, though that process has only just begun. Coalition officials have supported the effort because of concerns about the private forces, but say the alternative—the Afghan police—isn’t yet competent enough to take over the job.

The majority of the private security contractors are Afghan; companies employing them are both international and locally based. The Senate inquiry focuses on the role of Department of Defense contractors, but the State Department also employs private guards.

According to U.S. Central Command figures cited in the report, Afghanistan has more than 26,000 private security personnel, 90% of whom are working under U.S. government contracts or subcontracts.

Doug Brooks, the president of International Peace Operations Associations, a group that represents security firms, said the report highlights the difficulty in complying with contract requirements to provide local hires. “There’s not a huge amount of choice in the local hires they can use,” he said. “Where are they going to get guys who have never smoked hashish, who have never worked for the Taliban or who have never considered joining the Taliban?”

The investigation, quoting a Naval Criminal Investigative Service report, said “contractors have been known to shoot at Marines” and that Afghan security personnel were often “high on drugs” while at their posts.

In one case, Senate investigators found, a Marine was killed earlier this year by U.S.-funded security contractors who opened fire on a Marine foot patrol in Farah province.

In another example, the son of a man who provided staff for a guard force at a coalition facility was “suspected of being an agent of a hostile foreign government,” likely shorthand for Iran.

The inquiry singles out two security firms—ArmorGroup, a subsidiary of U.K.-based G4S PLC, and EOD Technology Inc., or EODT, of Lenoir City, Tenn.—for relying on dubious local power brokers, including individuals described in U.S. military reports as Taliban affiliates and criminals.

An ArmorGroup spokesman said the company “engaged workers from two local villages as stated by the Senate Report—but did so only upon the recommendation and encouragement of U.S. Special Forces.”

The company’s personnel “remained in close contact with U.S. Special Forces personnel to ensure that the company was constantly acting in harmony with, and in support of, U.S. military interests and desires,” the spokesman said.

EODT said in a statement it “has never been advised by the U.S. military” of problems with its hiring practices. The company said it has cooperated with the investigation and “stands ready to engage the U.S. military or other stakeholders about these issues in order to improve our internal processes and contract performance.”

A contractor interviewed by investigators described the local guard force recruiters as “straightforward 1920s Chicago.”

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