The world's largest retailer, U.S.-based Wal-Mart Stores, says it is expanding its internal investigation into whether company officials paid bribes to foreign officials to help expand the company's operations abroad.

A year after Wal-Mart revealed that U.S. officials are investigating its Mexican subsidiary, the company said Thursday in a filing with the U.S. Securities and Exchange Commission that it is also looking at its activities in Brazil, China, India and elsewhere.

U.S. law prohibits American companies from engaging in corrupt practices in other countries, such as paying bribes.