Hijackers Mohammed Atta and Abdulaziz al-Omari pass through security at Portland International Jetport in Maine. They board a shuttle flight to Boston's Logan International Airport, where they connect to Los Angeles-bound American Airlines Flight 11. Over the next hour and a half, seventeen other hijackers clear security checks at Logan Airport, Newark International Airport, and Washington Dulles International Airport. All presumably carry knives, box cutters or other concealed weapons on their person or in carry-on luggage. Although eight are randomly tagged for additional screening and a gate agent flags two as suspicious, none are prevented from boarding their intended flights.1

Still from a security camera at the Portland airport security checkpoint on September 11, 2001 at 05:53:37 showing Mohamed Atta2

1 “Hijackers Pass Through Security Screening” National September 11 Memorial & Museum n.d. Web. March 16, 2011 http://timeline.national911memorial.org/#/Explore/2/Entry/523
2 “Security Camera Atta” United States v. Zacarias Moussaoui, Criminal No. 01-455-A, Prosecution Trial Exhibits Web. n.d. April 27, 2011 http://www.vaed.uscourts.gov/notablecases/moussaoui/exhibits/prosecution/FO07021.jpg
Courtesy of the National 9/11 Memorial and Museum