Nearly half of the September 11, 2001, Islamic terrorist hijackers who flew planes into the World Trade Center and the Pentagon were able to remain in the United States, undetected, despite having overstayed their visas.

For 13 of the 19 terrorists, driver’s licenses were obtained after they arrived in the U.S. from Saudi Arabia, Egypt, the United Arab Emirates, and Lebanon. For seven of the terrorists, they were supposed to have been deported from the U.S. at some point before they could carry out the 9/11 attacks, as they had overstayed their visas either a year before the attack or during the time of the attack.

Subsequently, none of the terrorists were deported.

One of the terrorists, Nawaf al-Hamzi, for example, was able to obtain driver’s licenses in three different states, including California, Florida, and Virginia. Nawaf al-Hamzi, along with his terrorist brother, Salem al-Hamzi — who obtained a Virginia driver’s license — arrived in the U.S. on tourist visas in 1999 from Saudi Arabia.

Nawaf al-Hamzi arrived back in the U.S. in 2001 and lived in New York for just a couple months before hijacking American Airlines Flight 77 and flying the plane into the Pentagon. A month before the 9/11 attacks, Nawaf al-Hamzi was placed on a terrorist watch list.

Nawaf al-Hamzi should have been deported nine months before the attack, as he had overstayed his visa.

Mohamed Atta — who hijacked American Airlines Flight 11 and flew the plane into the World Trade Center’s North Tower — had overstayed his visa as of December 2000, but was not deported at the time. Atta was able to obtain a Florida driver’s license despite being arrested months before for driving without a license.

Atta’s name appeared on a terrorist watch list in January 2001 and he was detained in the United Arab Emirates before returning to the U.S. to carry out the 9/11 attacks.

Hani Hasan Hanjour, the terrorist pilot of American Airlines Flight 77 that was flown into the Pentagon, was considered an illegal alien at the time of the 9/11 attacks, as he did not enroll in school despite obtaining a student visa in September of 2000.

Satam al-Suqami and Waleed al-Shehri, likewise, had overstayed their visas in the U.S. before aiding in the Flight 11 hijacking and Twin Towers terrorist attack.

Marwan al-Shehhi — the terrorist responsible for flying United Airlines Flight 175 into the World Trade Center’s South Tower — took at least three trips out of the U.S. and back to the country despite having overstayed his visa as of November 2000. Al-Shehhi was also able to obtain a Florida driver’s license.

Ahmed al-Ghamdi, one of the Flight 175 hijackers, was in the U.S. illegally, as he had overstayed his visa but was never deported, allowing him to hijack the flight and aid in flying the plane into the South Tower in Manhattan.

In Fiscal Year 2017, nearly 630,000 foreign nationals overstayed their visas, as Breitbart News reported. As of October 2017, a total of about 607,000 of those foreign nationals were still in the U.S.