In what's become a grim and somber ritual for Americans, citizens will gather Tuesday to mark the 11th anniversary of the 9/11 terror attacks, in which Islamic extremists killed nearly 3,000 people.

Thousands are expected to come together in New York City, Washington, DC and Pennsylvania to pay their respects.

In New York, there were moments of silence at 8:46 a.m., 9:03 a.m., 9:37 a.m. and 10:03 a.m. to signal the impact of each hijacked plane. Additional silences were observed at 9:59 a.m. and 10:28 a.m. when the South and North Twin Towers fell. In addition, the families of the more than 2,750 killed at Ground Zero will read the names of their loved ones in memorial.

A ceremony will be held outside the Pentagon in Washinton, DC, where a hijacked plane crashed and killed more than 180 people. President Barack Obama and First Lady Michelle will observe a moment of silence on the White House lawn and later attend the Pentagon ceremony.

In Shanksville, PA, where United Airlines Flight 93 crashed, the names of 40 crew members and passengers were read starting at 10:03 a.m. by victims' families and volunteers who assisted with the crash. A bell will toll once for each name that's read.

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