Saudi-born Ibrahim Hassan Asiri, 28, considered a key figure in al-Qaida in the Arabian Peninsula, is now the chief suspect behind the mail bombs sent from Yemen and bound for the United States.
Together with U.S.-born cleric Anwar al-Awlaki, Yemeni militants and former Saudi inmates of Guantanamo, Asiri makes up the leadership of Al-Qaida in the Arabian Peninsula.
Asiri is also suspected of packing explosives into the underwear of a Nigerian accused of trying to blow up a Detroit-bound airliner last Christmas, and sent his own brother on a suicide mission against a top Saudi official last year. All the bombs contained an explosive known as PETN, or pentaerythritol tetranitrate.
Asiri and his brother served time in Saudi prisons, and moved to Yemen three years ago. Asiri is believed to be in Marib province, Yemeni officials say.
His most effective attack took place last year on top Saudi counterterrorism official Prince Mohammed bin Nayef, in which he recruited his younger brother, Abdullah, to pose as a repentant militant.
With the bomb hidden in a body cavity, Abdullah approached the prince and blew himself up. The prince was only wounded.