Mr Ashcroft: Balancing protection and immigration

The firm's chief executive and 30 other employees now face criminal charges.

Code-named "Operation Great Basin", the investigation lasted two years and involved 300 law enforcement officers from seven states.

It centred on Golden States Transportation, a bus company based in Los Angeles.

During the course of the investigation, said Mr Ashcroft, Golden State had illegally brought between 50 and 300 Mexicans every day into the United States.

It scheduled arrivals and departures to take place late at night, he said, and bus routes were constantly changed to avoid detection by the authorities.

Stark warning

Mr Ashcroft said the company's chief executive was one of 31 employees charged with trying to profit from the transportation and harbouring of illegal immigrants.

"Operation Great Basin serves as a warning to those organisations, companies or individuals who would smuggle illegal aliens for profit. United States law enforcement is ready and able to find you, to investigate you, and to prosecute you," Mr Ashcroft said.

The Attorney General said that in light of September's terror attacks, the administration was striking the right balance between protecting America's borders and maintaining a proud tradition of legal immigration.

This had been the largest ever such smuggling operation by a commercial company, he said.

Others too would be stopped from the border to the boardroom, he added.