Some flight crews have been prevented from entering the United States because their names too closely match those on the terrorist watch list, FOX News has confirmed.

An aviation security source confirmed that at least one foreign flight has been turned around because the names were not provided in advance.

Foreign carriers are required to provide the names to the Transportation Security Administration (search), which vets them through their National Risk Assessment Center (search) in Virginia. Though the TSA would like the names well in advance, the source said they usually get them only hours before a flight takes off.

While there is still concern about passengers who may attempt to hijack planes, the source told FOX News that in the last six months, there has been a significant development.

The intelligence reporting suggests foreign flight crews and pilots are the "possible or preferred means of attack" for groups like Al Qaeda (search).

It was reported late last year and early this year that U.S. officials were concerned that terrorists with flight training may have infiltrated foreign carriers or have been trained to take over planes. Those terrorists with flight training were put on a federal watch list.

Around Christmas time, several incoming international flights were canceled amid fear that Al Qaeda was trying to infiltrate the flights.

Fox News' Catherine Herridge contributed to this report.