Trump Remembers September 11th Attack VictimsDonald Trump has, for the first time as president, led the nation in remembering the September 11 attacks on New York and Washington in 2001. Trump marked the 16th anniversary of the worst act of terrorism on U.S. soil in a ceremony at the White House, with a moment of silence at 8:40 a.m., the time when a first airliner hijacked by al-Qaida terrorists crashed into the World Trade Center in New York. A second jet hit the second WTC tower 23 minutes later, and the huge skyscrapers collapsed in flames and smoke soon afterwards. In New York, names of the victims were being read out after a moment of silence was held. Trump also attended a remembrance ceremony at the Pentagon, which was hit and badly damaged by another of the four commercial jets seized on that fateful day. "The horror and anguish of that dark day were seared into our memory forever," Trump said in remarks in front of the Pentagon. Speaking about the victims, Trump said "We mourn them, we honor them and we pledge to never, ever forget them."\n\nWATCH: Trump on protecting US from future attacks No media source currently available Defense Secretary Jim Mattis said after the attacks of 2001, "Our nation rallied together as one." "Heroism and compassion were boundless," he said. WATCH: Mattis on US military preparedness No media source currently available Vice President Mike Pence traveled to Shanksville, Pennsylvania, where another hijacked plane crashed into a field. It is believed passengers tried to regain control of the aircraft once they realized their knife-wielding captors intended to steer it toward Washington for another attack, aiming for either the Capitol or the White House, and that the hijackers crashed the plane rather than surrender. In Photos: 9/11 anniversary Everyone aboard all four hijacked aircraft died on September 11, 2001. Including mass casualties in New York City and a heavy loss of life at the Pentagon, the attacks killed a total of about 3,000 people. The Islamist militant named by U.S. investigators as "the principal architect" of the 2001 attacks," Khalid Sheikh Mohammed, was captured in Pakistan early in 2003 and is held at the U.S. military prison in Guantanamo Bay, Cuba. The September 11 plot had been approved in advance by al-Qaida leader Osama bin Laden, who was shot to death by U.S. commandos in May 2011, during a raid on a walled compound in Pakistan where he was hiding.