The next month, the same group claimed responsibility for detonating an explosive device outside the U.S. Consulate.

CNN: Gunmen open fire on Italian consul's car in Benghazi

He and his brother, Tamerlan, are suspected of detonating two explosive devices at the Boston Marathon race April 15, leaving three dead and more than 175 people wounded.

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Word of the October alert was just the latest sign of the concerns U.S. officials have about the possibility al Qaeda or another terrorist network might gain access to a nuclear device -- either a nuclear weapon or a so-called "dirty bomb, " one that contains radioactive material spread by detonating a conventional explosive.

CNN: Nuclear weapon plot deemed not credible

Mr. MICHAEL CHERTOFF (Department of Homeland Security): The terrorists planned to carry the components of the bombs, including liquid explosive ingredients and detonating devices, disguised as beverages, electronic devices or other common objects.

NPR: It's Hard to Spot a Liquid Threat to Airline Safety