Mark Krikorian has stated that out of the 48 al-Qaeda operatives who committed crimes in the US between 1993 and 2001, 12 of them were illegal immigrants when they committed their crimes and seven of them were visa overstayers (including two of the conspirators in the first World Trade Center attack, one of the figures from the New York subway bomb plot, and four of the 9/11 terrorists).
Vice Chair Lee H. Hamilton and Commissioner Slade Gorton of the National Commission on Terrorist Attacks Upon the United States commented that of the nineteen hijackers in the September 11, 2001 attacks,
Two hijackers could have been denied admission at the port on entry based on violations of immigration rules governing terms of admission. Three hijackers violated the immigration laws after entry, one by failing to enroll in school as declared, and two by overstays of their terms of admission.
Since the attacks of September 11, 2001, illegal immigrants within the United States have attempted to carry out other terrorist attacks as well.
Three of the six conspirators in the 2007 Fort Dix attack plot—Dritan Duka, Shain Duka, and Eljvir Duka—were ethnic Albanians from the Republic of Macedonia who entered the United States illegally through Mexico with their parents in 1984.
Hosam Maher Husein Smadi, an illegal immigrant from Jordan who remained in the United States after the expiration of his tourist visa, was arrested in September 2009 for attempting to carry out a car bomb attack against Fountain Place in Dallas.
In the case of Rasmea Odeh, she was convicted of immigration fraud in 2014 for concealing her arrest, conviction, and imprisonment for a fatal terrorist bombing in Israel.
Such anecdotal evidence, while not statistically significant, is important to consider.