WASHINGTON -- A half dozen of the largest federal agencies squandered $19 billion in erroneous payments last year, and the total amount wasted is probably far greater, according to a report released Friday.

Improper Medicare payments accounted for more than half the money, according to the study by the General Accounting Office, the investigative arm of Congress. The government also paid out more than it should in housing subsidies and tax refunds.

The mistakes occurred when government officials paid people twice, miscalculated the amount individuals should receive or sent checks to people who weren't eligible, according to the report.

The improper payments were culled from the annual financial reports of agencies that accounted for most of the government's $1.8 trillion budget. But the actual amount wasted is likely to be billions more since hardly any agencies reveal these kind of mistakes, according to the General Accounting Office.

"Few agencies publicly report improper payment information such as improper payment rates, causes and strategies for better managing their programs to reduce or eliminate these payments," the report stated.

Sen. Fred Thompson, R-Tenn., said Congress should fix that problem.

"Public scrutiny is often the most effective tool in focusing agency managers' attention on certain issues," said Thompson, who ordered the investigation.