WASHINGTON -- A senior Senate Democrat said Thursday the Pentagon should consider barring Blackwater, now called Xe Services, from a new $1 billion deal to train Afghan police because of "serious questions" about the contractor's conduct.

The comments by Senate Armed Services Committee Chairman Carl Levin suggests thinning patience in Congress for the Pentagon's heavy reliance on contractors on the battlefield.

U.S. efforts to rebuild Iraq and Afghanistan using independent contractors has been a boon for companies like Blackwater and saved money and time for the Defense Department, whose forces are busy in combat.

But the outsourcing has made it more difficult for military commanders to control what happens on the battlefield.

In one recent incident in Afghanistan, two contractors tied to Blackwater allegedly killed two Afghan civilians and injured a third. U.S. officials say the May 2009 shooting damaged relations with the local population

"The inadequacies in Blackwater's performance appear to have contributed to a shooting incident that has undermined our mission in Afghanistan," Levin, D-Mich., wrote in a letter to Defense Secretary Robert Gates.

Blackwater, headquartered in Myock, N.C., changed its name to Xe Services after its security guards were accused of killing unarmed Iraqi civilians more than two years ago.

Mark Corallo, a company spokesman, said Xe Services agrees with Levin that the Pentagon should carefully review its past performance when deciding future contracts.

"We are confident that Xe's record of service in training thousands of security personnel in Afghanistan demonstrates the companys strong record of supporting critical U.S. government initiatives in Afghanistan, which are essential to advancing the United States national interest," he said in an e-mailed statement.

Pentagon spokesman Bryan Whitman said Thursday he knew of no effort under way to ban Xe Services from contracting with the military. Until then, the company would be legally allowed to submit a bid, he said.

In a separate letter to Attorney General Eric Holder, Levin called for a Justice Department investigation into whether Blackwater officials duped the Army into awarding a separate $25 million contract to train Afghan police by creating a shell subcontractor called Paravant.

Levin alleges that company officials boasted to the Army of its large presence overseas and several years of experience without mention of the Blackwater name or that the State Department had dumped the contractor in 2009 after saying it had lost confidence in its management.

Corallo said the contracting officials were aware that Paravant was a Blackwater subsidiary.

Xe is among five companies eligible to compete for a $1 billion contract to train Afghanistan's national police force. DynCorp International of Falls Church, Va., had held a large contract for such training since 2003.

But a decision to transfer control of the program from the State Department to the military is ending DynCorp's run and opening a major opportunity for Xe.

Xe has been shifting its work to training, aviation and logistics after the September 2007 incident at Nisoor Square in Baghdad. Its security guards were accused of killing 17 unarmed Iraqi civilians.