ABOARD AIR FORCE ONE -- President Barack Obama will mark the 11th anniversary of the Sept. 11, 2001, terrorist attacks with a series of commemorations in and near Washington.

Obama and first lady Michelle Obama will gather Tuesday morning on the White House lawn for a moment of silence. Afterwards, they'll attend a memorial at the Pentagon. In the afternoon, Obama will visit with wounded service members and their families at Walter Reed National Military Medical Center.

Vice President Joe Biden will be in Somerset County, Pa., for a service remembering the victims of Flight 93. In New York, Homeland Security Secretary Janet Napolitano will be at the World Trade Center Memorial.

White House spokesman Jay Carney announced the plans aboard Air Force One as Obama left North Carolina the morning after his convention speech.

Also on HuffPost:

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  • 7:59 a.m.

    The four airplanes that were hijacked on 9/11 <a href="http://www.reuters.com/article/2011/08/10/idUS221159963020110810" target="_hplink">began</a> taking off at 7:59 a.m. The first to depart was American Airlines Flight 11, a Boeing 767 that left Boston's Logan International Airport for Los Angles with 92 people on board. At 8:14 a.m., United Airlines Flight 175 -- a Boeing 767 with 65 passengers on board -- also left Logan for Los Angeles. American Airlines Flight 77 left Washington Dulles International Airport at 8:20 a.m. The plane, a Boeing 757 with 64 people on board, was bound for Los Angeles. Finally, at 8:42 a.m., United Airlines Flight 93 departed from Newark International Airport. The Boeing 757, which had 44 passengers that morning, was bound for San Francisco.

  • 8:46 a.m.

    The first crash <a href="http://www.nysm.nysed.gov/wtc_timeline/zoomify.html" target="_hplink">occurred</a> at 8:46 a.m. when Flight 11 hit the north tower of New York's World Trade Center. According to the <a href="http://www.9-11commission.gov/report/index.htm" target="_hplink">9/11 Commission Report</a>, two flight attendants contacted American Airlines as the plane was being hijacked to provide details of the emergency. They reported the use of Mace or a similar spray, several stabbings and a bomb threat. The last known communication from the plane came when flight attendant Madeline "Amy" Sweeney, on the phone with American Flight Services manager Michael Woodward, said, "Oh my God we are way too low."

  • 9:03 a.m.

    The second crash happened at 9:03 a.m., when Flight 175 hit the south tower of the World Trade Center. The last communication made with air traffic control was made at 8:42 a.m., but passengers were able to provide details of the flight by contacting their families by phone. <a href="http://timeline.national911memorial.org#/Explore/2/Entry/533" target="_hplink">Brian Sweeney</a> called his wife, Julie, to tell her the plane had been hijacked, and Peter Hansen <a href="http://www.9-11commission.gov/report/index.htm" target="_hplink">told</a> his father, Lee, "I think they intend to go to Chicago or someplace and fly into a building."

  • 9:05 a.m.

    President George W. Bush <a href="http://timeline.national911memorial.org/#/Explore/2" target="_hplink">learned</a> of the attacks at 9:05 a.m. while sitting in a second grade classroom at an elementary school in Sarasota, Florida. White House Chief of Staff Andrew Card informed him of the attacks, whispering into his ear during the students' reading lesson. Bush recently <a href="http://channel.nationalgeographic.com/channel/community/blogs/ngc/_george-w-bush-911-interview" target="_hplink">shared</a> his memories of that day with <i>National Geographic</i>. When he received news of the first plane crash at 8:50 a.m. -- just before entering the classroom -- he thought it was "a light aircraft, and my reaction was, man, the weather was bad or something extraordinary happened to the pilot." It wasn't until Card informed him of the second plane that Bush knew America was under attack.

  • 9:31 a.m.

    In an <a href="http://www.youtube.com/watch?v=QP1K84iRZPo" target="_hplink">address</a> from Emma Booker Elementary School in Sarasota, Florida, President Bush called the attacks "a national tragedy" and "an apparent terrorist attack on our country." "I have spoken to the vice president, to the governor of New York, to the director of the FBI, and have ordered that the full resources of the federal government go to help the victims and their families, and to conduct a full-scale investigation to hunt down and to find those folks who committed this act," Bush said. "Terrorism against our nation will not stand."

  • 9:36 a.m.

    At 9:36 a.m., Secret Service agents <a href="http://timeline.national911memorial.org/#/Explore/2" target="_hplink">evacuated</a> Vice President Dick Cheney and his aides from his office to the Presidential Emergency Operations Center, a Cold War-era bunker beneath the White House.

  • 9:37 a.m.

    Flight 77 crashed into Pentagon at 9:37 a.m. The 9/11 Commission Report tells how passenger Barbara Olson <a href="http://www.gpoaccess.gov/911/index.html" target="_hplink">called</a> her husband Ted -- the solicitor general of the United States -- to inform him of the attacks. She reported that the flight had been taken over and that the aircraft was "flying low over houses." A few minutes later, air traffic controllers at Dulles International Airport observed plane on their radar traveling at "a high rate of speed." Officials from Ronald Reagan Washington National Airport warned the Secret Service of the aircraft shortly before Flight 77 hit the Pentagon.

  • 9:45 a.m.

    At 9:45 a.m. -- minutes after Flight 77 crashed into the Pentagon -- the White House and U.S. Capitol were <a href="http://timeline.national911memorial.org/#/Explore/2" target="_hplink">evacuated</a>.

  • 9:59 a.m.

    After burning for 56 minutes, the south tower of the World Trade Center <a href="http://timeline.national911memorial.org/#/Explore/2" target="_hplink">collapsed</a> at 9:59 a.m. The fall, which killed approximately 600 workers and first responders, lasted 10 seconds.

  • 10:03 a.m.

    The fourth hijacked plane crashed at 10:03 a.m. in a field in Shanksville, Pennsylvania. The 9/11 Commission Report <a href="http://www.gpoaccess.gov/911/pdf/sec1.pdf" target="_hplink">says</a> several passengers made calls from the plane and received word of the other hijackings. Upon hearing the news that major cities were being targeted, the passengers decided to revolt: <blockquote>Five calls described the intent of passengers and surviving crew mem­bers to revolt against the hijackers. According to one call, they voted on whether to rush the terrorists in an attempt to retake the plane. They decided, and acted.<br><br>At 9:57, the passenger assault began. Several passengers had terminated phone calls with loved ones in order to join the revolt. One of the callers ended her message as follows:"Everyone's running up to first class. I've got to go. Bye."</blockquote> <a href="http://timeline.national911memorial.org/#/Explore/2" target="_hplink">According</a> to the 9/11 Memorial, the hijackers deliberately crashed in a field to prevent passengers from retaking the airplane. The crash site in Shanksville is approximately 20 minutes flying time from Washington, D.C.

  • 10:28 a.m.

    At 10:28 a.m., after burning for 102 minutes, the north tower of New York's World Trade Center <a href="http://timeline.national911memorial.org/#/Explore/2" target="_hplink">collapsed</a>, killing approximately 1,400 people.

  • 11:02 a.m.

    New York City Mayor Rudy Giuliani <a href="http://www.nysm.nysed.gov/wtc_timeline/zoomify.html" target="_hplink">ordered</a> an evacuation of lower Manhattan at 11:02 a.m., alerting everyone south of Canal Street to get out.

  • 1:04 p.m.

    At 1:04 p.m., after all American air space had been cleared, President Bush <a href="http://www.nysm.nysed.gov/wtc_timeline/zoomify.html" target="_hplink">addressed </a>the nation from Barksdale Air Force Base in Louisiana, informing citizens that the U.S. military "at home and around the world is on high alert status." "Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts," Bush <a href="http://www.youtube.com/watch?v=bwgqmaeV7o0" target="_hplink">said</a>.

  • 5:20 p.m.

    Hours after the attacks that morning, the 47-story 7 World Trade Center building <a href="http://www.nysm.nysed.gov/wtc_timeline/zoomify.html" target="_hplink">collapsed</a> from ancillary damage. No one was in the building at the time.

  • 8:30 p.m.

    President Bush gave his final <a href="http://articles.cnn.com/2001-09-11/us/bush.speech.text_1_attacks-deadly-terrorist-acts-despicable-acts?_s=PM:US" target="_hplink">address</a> of the day from the White House at 8:30 p.m. From the Oval Office, the president informed Americans that he had implemented federal emergency response plans, noting emergency teams and the military were already at work: <blockquote>Today, our fellow citizens, our way of life, our very freedom came under attack in a series of deliberate and deadly terrorist acts.<br><br>The victims were in airplanes or in their offices -- secretaries, businessmen and women, military and federal workers. Moms and dads. Friends and neighbors.<br><br>Thousands of lives were suddenly ended by evil, despicable acts of terror.<br><br>The pictures of airplanes flying into buildings, fires burning, huge structures collapsing, have filled us with disbelief, terrible sadness and a quiet, unyielding anger.<br><br>These acts of mass murder were intended to frighten our nation into chaos and retreat. But they have failed. Our country is strong. A great people has been moved to defend a great nation.</blockquote>