By Dave Graham

MEXICO CITY (Reuters) - Mexico's federal comptroller's office said on Wednesday it would open an investigation into allegations that the Mexican unit of Wal-Mart Stores Inc bribed officials to expand its business in Mexico.

The office said it has begun checking the federal paperwork and permits Wal-Mart de Mexico, known as Walmex, obtained to open and operate its stores in Mexico.

The comptroller's office added that it would ask U.S. authorities for information on the case, in order to carry out its investigation.

Wal-Mart did not immediately comment on the investigation.

The world's largest retailer is also facing a criminal probe by the Department of Justice over potential violations of the U.S. Foreign Corrupt Practices Act, a U.S. law that forbids bribing foreign officials.

On Saturday, The New York Times reported that a senior Wal-Mart lawyer received an email from a former executive at Walmex in September 2005 that described how the Mexican affiliate, known as Walmex, had paid bribes to obtain permits to build stores in Mexico. The newspaper said that senior Wal-Mart officials stymied an internal investigation into the alleged bribery.

In response to that story, "We have met voluntarily with the U.S. Department of Justice (DOJ) and the Securities and Exchange Commission (SEC) to self-disclose the ongoing investigation on this matter," Wal-Mart said on Saturday. "We also filed a 10-Q in December to inform our shareholders of the investigation. The Company's outside advisors have and will continue to meet with the DOJ and SEC to report on the progress of the investigation."

(Reporting by Dave Graham in Mexico City, Jessica Wohl in Chicago, additional reporting by Aruna Viswanatha, David Ingram and Andrew Longstreth; Editing by Gerald E. McCormick and Matthew Lewis)