On the 20th anniversary ceremonies to remember the attacks that occurred on 9/11 are being held  Saturday.

At the 9/11 Memorial in New York City, a solemn crowd of hundreds gathered as a bell tolled and a moment of silence was held at 8:46 a.m., the moment the north tower of the World Trade Center was hit 20 years ago.

On Sept. 11, 2001, 19 al-Qaida hijackers took control of four U.S. commercial airplanes in a plot to attack major U.S. landmarks. Two of the planes toppled the World Trade Center’s twin towers in New York. One crashed into the Pentagon in northern Virginia. Another, headed toward Washington, D.C., careened into a field in Pennsylvania after the passengers and crew fought back to take control.

The attacks prompted the war in Afghanistan weeks later, followed by the war in Iraq in 2003. They changed America’s domestic security and surveillance infrastructure.