A look at key events surrounding the Christmas Day attack on a U.S. airliner. Umar Farouk Abdulmutallab is accused of trying to detonate an explosive device hidden on his body as the plane approached Detroit on a flight from Amsterdam.

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2004 to 2005:

_Abdulmutallab is in Yemen for a year, learning Arabic at the Sana'a Institute of Arabic Languages.

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2005

_Abdulmutallab graduates from the British School of Lome in Togo. The month is unclear.

_He begins attending University College London in September.

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2008

_Abdulmutallab graduates from University College London in June.

June 12:

_He applies for a visa at the U.S. Embassy in London.

June 16:

_His visa is granted. The visa is a regular multiple-entry tourist visa valid until June 12, 2010.

Aug. 1-17:

_Abdulmutallab attends two-week seminar in Houston at AlMaghrib Institute, an Islamic education center.

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2009

January to about July:

_Abdulmutallab attends the University of Wollongong in Dubai.

May:

_Britain refuses to grant Abdulmutallab a student visa because the school on his application form was not a government-approved institution.

August to early December:

_Abdulmutallab visits Yemen after receiving a visa to study Arabic at a school in Sana'a, according to the Yemeni Foreign Ministry. He spends at least part of the time studying Arabic, investigators say. Students and administrators have described him as friendly and outgoing, with no overtly extremist views. Yemeni authorities are looking into Abdulmutallab's frequent visits to a mosque in the old, historic part of the city and the people he was with during his stay.

October

_Abdulmutallab's father goes to Nigerian authorities to express concerns about his son.

Nov. 13:

_A man tries to board a commercial airliner in Mogadishu, Somalia, carrying powdered chemicals, liquid and a syringe that could have caused an explosion. The case bears similarities to the plot to blow up the Detroit-bound airliner. The Somali man _ whose name has not yet been released _ was arrested by African Union peacekeeping troops before the Daallo Airlines flight took off. It had been scheduled to travel from Mogadishu to the northern Somali city of Hargeisa, then to Djibouti and Dubai.

Nov. 19:

_Abdulmutallab's father goes to the U.S. Embassy in Abuja, Nigeria, to express concern his son was in Yemen and had fallen under the influence of extremists.

Nov. 20: