Roughly six million illegal immigrants currently in the United States arrived the same way the 19 9/11 hijackers did: by overstaying their visas.
Per Breitbart:
All nineteen 9/11 terrorists — who murdered nearly 3,000 Americans and injured more than 6,000 others in 2001 — arrived in the U.S. legally, with 16 obtaining tourist visas and three others obtaining business and tourist visas.
In total, seven of the 19 terrorists overstayed their visas at some point either before the 9/11 attacks or at the time of the attacks and were supposed to be deported, but never were.
…
Eighteen years after the 9/11 terrorist attacks, the loophole where legal immigrants become illegal aliens after overstaying their visas remains fully open, with at least 4.5 to six million foreign nationals living in the U.S. who should have been deported after their visas expired, according to Pew Research Center.
In total, there are roughly 11 to 22 million illegal aliens living in the U.S. at any given moment, and more than 1.2 million foreign nationals are legally admitted to the country every year — like all 19 terrorists were.
The 9/11 Commission Report suggested the Department of Homeland Security implement a nationwide biometric entry-exit system to track immigrants who overstay their visas.
No such action has ever been taken.