Congressional investigators are struggling to figure out how federal agencies shelled out $100 billion in payments last year to people who where not entitled to receive them.
Congressional investigators warn the figure could be even higher.
"The federal government is unable to determine the full extent to which improper payments occur and reasonably assure that appropriate actions are taken to reduce them," Beryl H. Davis, director of financial management at the Government Accountability Office, said in prepared testimony for a House hearing Wednesday.
In a hearing Wednesday before the House Oversight Government Operations Subcommittee lawmakers called the figures "staggering."
"It's over $100 billion each of the last five years. That's a staggering half a trillion dollars in improper payments," Committee Chairman John Mica, R-Fla., said.
Medicare programs account for half of the mismanaged money.
But there were also tax credits given to families that didn't qualify, unemployment benefits given to people who had jobs, and medical payments made for unnecessary treatments.
"The fact is, improper payments are staggeringly high in programs designed to help those most in need - children, seniors, and low-income families," Mica said.
The White House said it has stepped up efforts to identify the cause of the errors.