Wal-Mart, the world's biggest retailer, is facing a federal grand jury investigation into possible immigration law violations.

The company said on Tuesday that it has received a letter from prosecutors naming it a "target" of the investigation.

The action follows last month's raids on 61 Wal-Mart stores across the United States that brought 250 arrests. Those in custody came from 18 countries and worked for companies hired by Wal-Mart to clean its stores.

U.S. officials have said employers who knowingly hire illegal migrants could face fines of more than $10,000 per worker.

Wal-Mart says it is cooperating with the investigation and denies it has broken any laws.