Wal-Mart insiders turned a blind eye, officials say

The US Immigration and Customs Enforcement (ICE) Bureau, part of the new Department of Homeland Security, said the immigants were hired by a contract cleaning firm, and that Wal-Mart executives had turned a blind eye to the practice.

The ICE's investigation, known as "Operation Rollback", targeted workers at 61 Wal-Mart stores in 21 states.

Investigators are still pursuing at least 50 workers.

Inside knowledge

According to ICE, some Wal-Mart officials had direct knowledge of the recruitment of illegal immigrants.

Federal agents have reportedly conducted searches at Wal-Mart's Arkansas headquarters.

And federal grand jury subpoenas have been issued for the Wal-Mart executives to testify.

Wal-Mart said it was "committed to cooperating" with federal officials, and stressed that it was not directly responsible for the behaviour of third-party contractors.

The probe is another stroke of bad news for Wal-Mart, the world's biggest retailer.

The firm already faces a wave of lawsuits alleging sexual discrimination, and has been criticised for its apparently anti-union stance.