Could the Sept. 11 terrorist attacks have been prevented? That's a question that should weigh heavily on the heart of anyone who might have been in a position to stop them. As an eight-year member of the Senate intelligence committee, I recently told the Sept. 11 commission that the answer is probably yes. And the attacks could happen again.

Fifteen of the 19 men who flew hijacked airliners into the World Trade Center, the Pentagon and the Pennsylvania countryside that day in 2001 were natives of Saudi Arabia. All had entered the United States legally, using visas that should never have been granted and that were in blatant violation of the spirit and the letter of U.S. immigration law.

The Immigration and Nationality Act makes clear that applicants for nonimmigrant visas are to be considered ineligible until they demonstrate otherwise. In other words, the burden of proof is on the applicants to show that they do not intend to overstay or otherwise violate the terms of their visa. All applicants are required to provide specific information, such as their identity, travel plans, financial resources and destination. This system was created for a good reason: to discourage illegal immigration by preventing the entry of those with no intention of leaving the United States.

But for Saudi nationals, we looked the other way. Because Saudi Arabia is a rich country -- and, perhaps more important, because it is considered a U.S. ally -- before Sept. 11, 2001, the assistant secretary of state for consular affairs at the time, Mary Ryan, had relaxed the visa application rules, even to the point of implementing "Visa Express," an unprecedented program that literally delegated visa processing to travel agents to speed it up. The basic legal requirement that applicants provide accurate and persuasive information was ignored.

The problems with the terrorists' applications weren't trivial, such as punctuation or spelling. Omitted information included home addresses, means of financial support and travel plans while within the United States. Only three forms listed a "Name and Address of Present Employer or School," and only one of the 15 applicants listed an actual destination address in the United States. The rest of the hijackers put down vague locations such as "California," "New York" and "Hotel." One simply wrote "No." Only two were given oral interviews, which are common for applicants from poorer or less stable countries or those whose written applications require clarification. In fact, the Sept. 11 commission found that two other suicide volunteers were turned down because al Qaeda feared they would have trouble, as natives of Yemen, getting visas of their own.

It's not as if Saudis should have been above suspicion. Saudi Arabia is known to harbor violent Muslim radicals; it is the home of Wahhabism, a particularly virulent and anti-American strain of Islam. The State Department should have been giving Saudi visa applications an extra level of scrutiny, not expediting them.

Unfortunately, the State Department does not seem to have learned much from the experience. It continues to maintain that its pre-Sept.11 visa policies in Saudi Arabia were reasonable because of a lack of specific intelligence indicating otherwise. It is true that none of the 15 applicants was on a specific "watch list" at the time. But they still would have been stopped in their tracks if the State Department's Bureau of Consular Affairs had simply followed even the minimum requirements of the law with regard to their applications.

Astonishingly, even after the Sept. 11 attacks the State Department adamantly refused to tighten visa procedures, waiting 10 months to begin interviewing all nonimmigrant applicants between the ages of 12 and 70, including Saudi citizens. Even today consular officers receive only a day or two of law enforcement training -- hardly enough to even begin to learn the art of identifying security risks. Nearly three years later, it's still far too easy for a terrorist to get a visa to enter the United States.

As important as it was for the Sept. 11 commission to examine the intelligence and air defense failures before the attacks, it is equally imperative to recognize that simple enforcement of immigration law might have prevented at least some of the attacks of that tragic day, and could stop new ones, if only a stubborn bureaucracy would admit its mistakes and fix them.

The writer is a Republican senator from Arizona and chairman of the Senate Judiciary subcommittee on terrorism and homeland security.