From this:

Thousands of foreign student pilots have been able to enroll and obtain pilot licenses from U.S. flight schools, despite tough laws passed in the wake of the 9/ll attacks, according to internal government documents obtained by ABC News.

"Some of the very same conditions that allowed the 9-11 tragedy to happen in the first place are still very much in existence today," wrote one regional security official to his boss at the TSA, the Transportation Security Administration.

"Thousands of aliens, some of whom may very well pose a threat to this country, are taking flight lessons, being granted FAA certifications and are flying planes," wrote the TSA official, Richard A. Horn, in 2005, complaining that the students did not have the proper visas.

Under the new laws, American flight schools are only supposed to provide pilot training to foreign students who have been given a background check by the TSA and have a specific type of visa...

Presumably, almost all would appear to have at least arrived legally, although if they had then violated the terms of their visas they would have presumably become illegal aliens, the same situation as some of the 9/11 hijackers were involved in.

The DHS appears to be scrambling to get out some sort of official response. That should be interesting.