Sen. Claire McCaskill wants some answers from Defense Secretary James Mattis on the Pentagon’s efforts to curb so-called improper payments to contractors and individuals.

McCaskill sent Mattis a letter Wednesday seeking an update on reports of inadequate Pentagon compliance with a law that mandates monitoring overpayments and underpayments, which totaled $1.2 trillion across the federal government between 2003 and 2016, according to the Government Accountability Office.

“When it comes to defense spending, it’s especially important that taxpayer dollars aren’t being wasted and instead are being used to strengthen our national security as much as possible,” said the Missouri senator, who is the top Democrat on the Homeland Security and Governmental Affairs Committee and a former Missouri state auditor, in a statement for CQ Roll Call. “The Department of Defense needs to lay out what they’ll do to identify and reduce improper payments moving forward.”

In May, the Pentagon’s inspector general issued a report identifying shortfalls in compliance with five of the six reporting requirements of the Improper Payments Elimination and Recovery Act, or IPERA, which requires agencies to find payment problems, devise solutions and report on progress.

The Defense Department either reported late or not at all on how it was doing curbing improper payments, the IG found.

Despite these problems, the Defense Department defended its record in a response included in the IG report.

“We consider the improper payments program, as a whole, to be fundamentally sound, and senior leadership remains engaged and committed to improving the program,” wrote Mark Easton, the Pentagon’s deputy chief financial officer, in an April 20 letter to the IG.

McCaskill’s new letter to Mattis calls for “an update on DoD’s status of implementing OIG’s recommendations to reform agency compliance with IPERA for FY 2017, including whether the stated deadlines in DoD’s action plan have been met, are on track for completion, or have been adjusted.”

McCaskill wrote that she also wants to learn about “any other steps, beyond implementing OIG’s recommendations, that DoD has taken to address and reduce improper payments.”

The Government Accountability Office issued its own report last May documenting government-wide noncompliance with the law. The Pentagon was out of compliance each year from fiscal 2012 through 2015, the GAO report said. In fiscal 2016, the Pentagon had nearly $1 billion in improper payments, nearly half of them in the Department of Defense Travel Pay.