The U.S. government today abruptly shut its consulate in Nuevo Laredo, Mexico, announcing an investigation into whether U.S. personnel there were selling visas.

The unusual step of halting operations at one of the busiest U.S. consulates was taken because the allegations of the sale of passage into the United States were felt to be particularly serious at a time when the Bush administration is trying to tighten border controls in the wake of the Sept. 11, 2001, terrorist attacks.

"We have begun an aggressive investigation into allegations of visa fraud," said Tony Garza, the U.S. ambassador here. "We will not tolerate fraudulent activity in the processing of documents for entry into the U.S. from Mexico."

Mexican police officials said they learned in November that Mexican citizens who had visited the Nuevo Laredo consulate were approached with illegal offers to buy visas. The Mexican authorities passed on that information to U.S. officials, they said. The Justice Department has not yet charged or arrested anyone in the case.

Tourist visas and "laser visas" for border residents -- a new scannable border-crossing card that is supposed to be tamper-proof -- are the main types of visas that officials believe may have been illegally sold. The visas, which cost $100 at the consulate, can fetch thousands of dollars on the black market.

A multimillion-dollar counterfeit document industry has long existed on the Mexican side of the border. But U.S. officials have been more vigorously scrutinizing documents and allegations of illegal entry since the Sept. 11 terrorist attacks.

U.S investigators are assembling a list of people who received illegal visas. Those names are being placed on a lookout list and distributed to law enforcement agencies, said the State Department spokesman, Richard Boucher.

More than three dozen employees -- U.S. citizens and Mexican local hires -- work at the consulate, just across the border from Laredo, Tex. According to U.S. officials quoted by Notimex, the Mexican news service, no consulate employee is allowed to leave the city during the investigation.

Another U.S. official in Mexico said "a large number of people on the inside may be involved" and that "a large number of visas may have been illegally issued." More than 117,000 visas were issued in Nuevo Laredo last year.

Staff writer Peter Slevin in Washington contributed to this report.