WASHINGTON (Reuters) - Two Sept. 11 hijackers studied at a Florida flight school long before their student visas were approved, the head of the U.S. immigration service said on Tuesday, in an admission of problems at the agency.James Ziglar, commissioner of the Immigration and Naturalization Service, told reporters, "I would have to say it is an understatement that INS's procedures ... were clearly not the most efficient and certainly not the most logical."Ziglar has been under fire since last week, when the INS informed the flight school the student visas had finally been approved. But he added in a...