Qassim al-Rimi, the military chief of Al-Qaeda in the Arabian Peninsula, said making bombs such as the ones used in the twin blasts in Boston in April, is within "everyone's reach".
"The Boston events... And the poisoned letters (sent to the White House), regardless of who is behind them, show that your security is no longer under control, and that attacks on you have taken off and cannot be stopped," he said, in the message entitled: "A letter to the American people."
Rimi said the killing of Al-Qaeda's founder Osama bin Laden in May 2011 and top Yemeni-American cleric Anwar al-Awlaki in September 2011, had not ended the struggle.
"Have you eliminated the jihadist groups that have spread everywhere after they had only been in Afghanistan? Today, they are in your land or close to it," he warned.
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To the Muslims in the United States, he said: "We encourage you to carry on with this way, be steadfast in your religion.
Two brothers, 19-year-old Dzhokhar and 26-year-old Tamerlan Tsarnaev, are accused of being behind the April 15 attack near the finish line of the Boston Marathon that killed three people and wounded more than 260.
Tamerlan Tsarnaev was killed in a shootout with police, and Dzhokhar was captured and taken into custody after an extensive manhunt.
At about the same time, three letters laced with ricin were discovered, one addressed to US President Barack Obama, one to Republican Senator Roger Wicker of Mississippi and a third to a justice of the peace in the same US state, Sadie Holland.