ASHINGTON, March 12 — Six months after Mohamed Atta and Marwan al-Shehhi crashed hijacked airliners into the World Trade Center, the Immigration and Naturalization Service sent out a routine notice this week telling a flight school that the two men had been approved for student visas to study there.

Embarrassed by the disclosure that the school had received the notifications on Monday, six months to the day after the attacks, the agency said it approved student visas for the two men last summer, before either had been identified as suspected terrorists.

After Sept. 11, the agency said, it failed to halt the normal process of having a subcontractor notify the school, Huffman Aviation in Venice, Fla., that it had authorized the student visas for the two, who are now among the world's most infamous terrorists.

The mistake was another embarrassing gaffe for an agency that has long been criticized in Congress for sloppy management and inept record keeping and for being unable to control the borders or keep track of foreigners in the United States legally or illegally.

A spokesman for the immigration agency, Russ Bergeron, said it regretted that it had not notified the subcontractor before it sent the letters last week. Mr. Bergeron said he knew of no other letters related to any other hijackers.

Associated Press Rudi Dekkers, owner of Huffman Aviation, holding the eligibility certificates for nonimmigrant student status on Tuesday in Venice, Fla. Topics  Alerts Atta, Mohammed Immigration and Naturalization Service Airlines and Airplanes Terrorism Create Your Own | Manage Alerts Take a Tour Sign Up for Newsletters

"We certainly regret that our contractor was not notified after Sept. 11," he said. "It was our responsibility to notify the contractor that the notifications were not needed in this case."

In a draft statement, the agency acknowledged that its system was flawed. But those admissions were deleted from the final statement issued this evening. The draft statement said, "This incident also highlights the need to replace a student information collections systems that I.N.S. has long stated was antiquated, inaccurate, untimely and of little utility to I.N.S., school or students."

The error seemed particularly difficult to explain, because Mr. Atta and Mr. Shehhi were among the most infamous of the 19 hijackers. The authorities say they believe that Mr. Atta, the suspected ringleader of the operation, was at the controls of American Airlines Flight 11, which struck the north tower of the trade center to begin the attacks. Mr. Shehhi was the suspected pilot on United Airlines Flight 175, which slammed into the south tower.

Representative John Conyers Jr. of Michigan, the senior Democrat on the House Judiciary Committee, said he was astonished at the slipshod handling of the visas.

"I have been wondering for some time whether this administration suffers from myopia as it continues with a misguided focus in pursuit of homeland security," Mr. Conyers said. "I am astonished that while the I.N.S. is fixated on detaining and rounding up countless Arab-Americans without any justification, it has failed to take basic steps to ensure that visas are not issued to known terrorists."

The flight school filed the requests for M-1 student visas for the two men in August 2000. The requests sought to change the men's visas from nonimmigrant visitor to student to let them take a $27,000 yearlong professional course.

The agency approved the requests on July 17, 2001, in Mr. Atta's case, and on Aug. 9 in Mr. Shehhi's. But CNN, which first reported the agency's action today, said the letters were postmarked last week.

In its statement, the agency said: "It is important to emphasize that the decisions regarding the request to change status were made in the summer of 2001, prior to the tragic events of Sept. 11. It is equally important to recognize that when the applications were approved, the I.N.S. had no information indicating that Atta or Shehhi had ties to terrorist organizations."

Letters to the two men would have been generated automatically and sent to them shortly after the visas had been approved. The flight school would have been notified as a follow- up.

The letters to Huffman Aviation were sent from the I.N.S. Student Processing Center, operated by ACS Inc., a contractor in London, Ky. The company has processed arrival and departure forms for foreign students, business visitors and travelers.