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It was just after 8:45 a.m. ET on Sept. 11, 2001, when the first jet struck the World Trade Center in New York City and the worst terrorist attack in the nation's history began. Nearly 3,000 people died.

At that time this morning, many Americans paused for a moment of silence. President Obama and first lady Michelle Obama were among them.

The White House, which webcast this morning's marking of the solemn anniversary by the president, says he will also visit the Pentagon for a 9:30 a.m. ET ceremony to remember those who died there 12 years ago. That too will be webcast.

At ground zero in New York City, family members of those killed in both the 2001 and 1993 World Trade Center attacks have been invited to read their loved ones' names once again. There's a webcast of that ceremony here, starting at 8:39 a.m. ET.

There will also be a ceremony at the Flight 93 National Memorial in southwestern Pennsylvania. Ground was broken there Tuesday for a new visitors center.

The White House also asks Americans to take part in the "September 11th National Day of Service and Rememberence."

Update at 8:48 a.m. ET. White House Ceremony:

At 8:45 a.m. ET, the Obamas, Vice President Biden and Jill Biden walked from the White House on to the South Lawn. They were trailed by a military honor guard and joined on the lawn by members of the White House staff.

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