By CHRISTINE HAUSER and DAVID STOUT
 
ederal officials arrested more than 300 illegal workers at 61 Wal-Mart stores around the country today after an investigation that focused on janitorial crews employed by outside contractors.

The raids were carried out as the workers — immigrants from numerous countries — finished their night shifts, said Garrison Courtney, a spokesman for the bureau of Immigration and Customs Enforcement. He said an office of a Wal-Mart executive whom he did not identify had also been searched.

Mr. Courtney said bureau agents acted as a result of an investigation begun several years ago by the United States attorney's office in Philadelphia.

If there is sufficient evidence that Wal-Mart executives or the contractors knowingly used illegal labor, Mr. Courtney said, "obviously there is a possibility there could be criminal charges." He said a company that knowingly uses illegal workers can be fined as much as $10,000 per worker. He also said the arrested workers would go before immigration judges and could face deportation. It was not immediately known if any contractors were among those arrested.

Mr. Courtney said much of the investigation focused on forms known as I-9's, which employers are required to use to verify the employment eligibility of their workers.

A Wal-Mart spokeswoman said the company was still assessing the details of the investigation. "We first learned about the raids when store managers at affected stores began calling us," said Mona Williams, the vice president for communications at Wal-Mart.

Ms. Williams said that Wal-Mart, the nation's No. 1 retailer, uses more than 100 third-party contractors to perform cleaning services in more than 700 stores nationwide. "We do not know if the current investigation involves one or more multiple contractors," she said. Nor could Mr. Courtney immediately shed light on that aspect of the case.

Ms. Williams, speaking from Wal-Mart's base in Bentonville, Ark., said that "we do require that each of the contractors uses only legal workers," though she said she did not know if Wal-Mart verifies that contractors adhere to that policy.

Responsibility for immigration issues were long the province of the Immigration and Naturalization Service, which was recently renamed and transferred to the new Department of Homeland Security.

Referring to federal immigration agents as they were formerly known, Ms. Williams of Wal-Mart said: "We are talking to the I.N.S. and are committed to cooperating with them. As I understand it, I.N.S. agents had come into the stores and were arresting members of the cleaning crews and taking them out of the store."

The raids were carried out in Alabama, Arkansas, Arizona, Connecticut, Delaware, Kentucky, Massachusetts, Maryland, Michigan, North Carolina, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, South Carolina, Tennessee, Texas, Virginia and West Virginia.

Wal-Mart employs a total of 1.4 million people in the United States and overseas, and had $245 billion in revenue last year, equaling 2.5 percent of the United States gross domestic product. Each week 138 million shoppers visit Wal-Mart's 4,750 stores. Last year, 82 percent of American households bought at least one item there.

Steven Greenhouse contributed reporting to this article.