As the world's largest retailer, Wal-Mart ought to set the standard for being above-board in its hiring practices. That's why discoveries in this week's 21-state raid on Wal-Mart by federal agents are troubling.

Agents arrested more than 250 illegal immigrants working at Wal-Mart stores as janitors, including two stores in Ohio. Wal-Mart quickly shifted blame to subcontractors who hired the illegal immigrants, saying that the company requires all subcontractors to hire legal residents.

A company that prides itself on knowing when any of its 4,750 stores needs to stock more boxes of Tide surely knows with whom it is doing business when it comes to labor.

It is Wal-Mart's responsibility to make sure its contractors get it right and abide by the law.

A federal official told the New York Times that a grand jury is investigating the incident and that the U.S. government believes Wal-Mart officials were aware of the use of illegal immigrants. Wal-Mart executives and contractors were wiretapped by the government, the official said.

If this is true, then Wal-Mart could face criminal charges and fines up to $2.5 million. But even if criminal charges are not filed, the practice of American companies hiring illegal workers in the name of cheap labor remains a matter of grave concern.

Americans flock to Wal-Mart because it boasts "everyday low prices" and features hundreds of thousands of products to choose from.

But if part of the reason those prices are so low is that the company turned a blind eye to the hiring of illegal workers, then it must be held accountable.