The suspected mastermind of last week's assault on the U.S. Embassy in Yemen is a longtime Al Qaeda operative who escaped from a Yemeni prison more than two years ago, according to U.S. national-security officials.

Nasir al-Wahishi, a former bodyguard to Osama bin Laden, is believed to have organized the well-coordinated Sept. 17 attack, according to two U.S. national-security officials, who requested anonymity when discussing sensitive information. Two vehicles, one of them carrying militants armed with automatic weapons and grenade launchers, tried to breach the heavily fortified walls of the American Embassy in Sanaa, the Yemeni capital, according to news reports from the region.

Seventeen people—including an 18-year-old American citizen from Lackawanna, N.Y.—were killed in the assault, making it the most deadly terrorist attack on a U.S. government facility since September 11, 2001. This week, Yemeni security forces announced they had arrested six people for complicity in the attacks, including an Islamic militant who had claimed responsibility for the assault in an Internet posting.

But one of the U.S. officials who spoke to NEWSWEEK said information, gathered since the attack, suggests Wahishi directed or instigated the plot involving the six men who were arrested. The official declined to specify precisely what the evidence was but acknowledged it was not a "smoking gun." If Wahishi's role is confirmed, it is likely to significantly change the way U.S. officials view the attack and further exacerbate tensions between the Bush administration and the Yemeni government headed by President Ali Abudullah Saleh.

A veteran Al Qaeda fighter, trainer and bodyguard, Wahishi pledged bayat (a loyalty oath) to bin Laden and served with him in Afghanistan in the days before 9/11, according to current and former U.S. officials. He later is believed to have assumed command of Al Qaeda operations in Yemen after Qaed al-Harithi, Al Qaeda's previous chief in Yemen, was killed in 2002 by a CIA-operated drone. In June 2007, a tape was released announcing Wahishi as the leader of Al Qaeda in Yemen. Later that summer, the Yemeni government announced that it suspected him of being responsible in part for an attack that killed a group of Spanish tourists.

Wahishi "is very much part of the inner circle of Al Qaeda," said Ali Soufan, a former FBI agent who worked on terrorism investigations in Yemen. "He would never conduct an attack if he did not get approval from bin Laden or people appointed by bin Laden."