Throughout the day, a U.S. Navy reconnaissance plane circled Yemen's capital, scouring the streets and alleyways below, searching for terrorist operatives behind a plan that U.S. intelligence officials say involves explosive-laden truck bombs being sent to strike the U.S. Embassy in Yemen or other Western targets.

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Nasir al Wuhayshi, who runs perhaps the most dangerous branch of Al Qaeda, is believed to be behind the plot, intelligence officials tell ABC News.

The hardened al Qaeda leader is said to be determined to strike beyond the borders of Yemen to the American homeland. He also is the suspected mastermind behind the plan to bring down a U.S. aircraft with an underwear bomb.

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And authorities fear al Wuhayshi just might get help from a large number of maximum-security prisoners, many of them al Qaeda members, who have been broken out prisons throughout the Middle East in recent weeks with the help of heavily armed militants.

Interpol reports there have been at least nine major prison breaks in just the last month, including in Iraq, where 500 were freed two jailbreaks in Iraq. In Libya, more than 1,000 prisoners, some of them suspected terrorists, were freed from a jail in Benghazi. And in Pakistan, a Taliban jailbreak freed nearly 250 convicts.

"It's possible they could be suicide bombers or become combatants," said Michael Chertoff, former secretary of the Department of Homeland Security. "Authorities are concerned some may have bomb-making capabilities as well."

Wuhayshi himself is a prime example of the threat. He broke out of a maximum security jail in 2006 in Yemen and was running Al Qaeda in the Arabian Peninsula just a few years later.

It is because of that concern that the reconnaissance plane spent so much time over the Yemeni capital.

"It [the plane] was buzzing over Sana'a for at least five hours," said Iona Craig, a freelance journalist in the city. "Then it was a break for maybe two hours, then it was back again."

After the State Department ordered the evacuation of all non-essential personnel from Yemen because of an increased terror threat, a U.S. military cargo plane this morning helped evacuate staff from the US Embassy.

The U.S. Air Force airlifted almost all of the U.S. personnel via a C-17 aircraft to Germany, leaving behind only the most essential employees.

Jen Psaki, a State Department spokeswoman, called the airlift from Sana'a an "order of reduction" of emergency personnel and said there were no immediate plans to evacuate private citizens.

"We will be evaluating the situation day-by-day and making staffing decisions accordingly," she said.

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ABC News' Martha Raddatz and Dana Hughes contributed to this piece.