As a result of the Visa Waiver program the British Bombers could visit the U.S. with a valid passport and no Visa. This is from Andrew Cochran. Andrew also says:

< ....>every year thousands of passports from countries that participate in the Visa Waiver Program are stolen or are reported missing.

And think about this:

< ....>The visa application can provide special agents conducting an investigation of a criminal or terrorist alien with a wealth of information. I often would attempt to secure these applications during the course of conducting investigations when I worked for the former INS as a special agent. Finally, the law makes visa fraud a crime. When such fraud is committed in conjunction with drug trafficking the penalty can be as much as 20 years of incarceration. When the fraud is carried out in support of terrorism, the penalty increases to a maximum of 25 years in jail. However, when an alien enters the United States in conjunction with the Visa Waiver Program no such prosecution is possible. Often the easiest law violation to prosecute where terrorists and criminal are concerned are immigration law violations.

There are those who will say that the visa requirement creates an inconvenient situation for would-be tourists. Yet, our nation appears to not be concerned about inconveniencing American citizens within their own cities in the United States when we have our backpacks and briefcases searched by police officers.< ...>

The Counterterrorism Blog: Michael Cutler: How Many British Bombers Might Have Come Here Without a Visa?