The Pentagon has done little to collect at least $100 million in overcharges paid in deals arranged by corrupt former officials of Kellogg Brown & Root, the defense contractor, even though the officials admitted much of the wrongdoing years ago, two senators have complained in a letter to Defense Secretary Robert M. Gates.

The letter also said that the Army had almost completely failed to move away from the monopolistic nature of the logistics contract that has paid the contractor, now called KBR, $31.3 billion for logistics operations in Iraq, Kuwait and Afghanistan.

The New York Times obtained a copy of the letter, dated Friday, by the senators, Claire McCaskill, Democrat of Missouri, and Susan M. Collins, Republican of Maine. Senator McCaskill is chairman of a contracting oversight subcommittee of the Senate Homeland Security and Governmental Affairs Committee, and Senator Collins is the subcommittee’s ranking Republican.

Their letter is likely to revive allegations that the Pentagon has become so close to KBR, and relies so heavily on it, that there is little inclination or incentive to discipline the company, in response to either Congress or critics outside the government.