WASHINGTON -- Hunting al-Qaida targets from Pakistan to Afghanistan, Yemen to Somalia, the fleet of U.S. armed Predator and Reaper drones that killed two American members of al-Qaida in Yemen are the night stalkers of the expanded U.S. war on terrorists.

Drones are often called the weapon of choice of the Obama administration, which quadrupled drone strikes against al-Qaida targets in Pakistan's lawless tribal areas, up from less than 50 under the Bush administration to more than 220 in the last three years.

They were mentioned in the writings of Osama bin Laden as the single most damaging weapon being used against his organization.

Drones were the first line of attack on Friday in Yemen, as U.S. spy planes and satellites shadowed the convoy of American-born cleric Anwar al-Awlaki, and fellow American Samir Khan, the co-editor of al-Qaida in the Arabian Peninsula's English-language recruiting magazine.

U.S. jets followed in reserve, in case the drones were unable to kill al-Awlaki in the first pass, as they had on May 5. Then, the dust cleared after the initial drone strikes to reveal al-Awlaki's convoy racing away unscathed. This time the drones killed four al-Qaida suspects.

Late Friday, two U.S. officials said that one of the four was the top al-Qaida bomb-maker in Yemen.

Ibrahim al-Asiri is the bomb-maker linked to the bomb hidden in the underwear of a Nigerian man accused of trying to blow up a plane over Detroit on Christmas Day 2009.

The unmanned drones, controlled from bases that can be as far away as the U.S., have a sinister look, with a distinctively long body, a wing span of up to 50 feet, a payload of Hellfire missiles tucked under the wings and another set of X-wings behind the main ones.

They can fly undetected at heights of up to 50,000 feet, far above that of most manned aircraft. They also can hover for hours.

The ones flown over Yemen, by the CIA and the U.S. military's Joint Special Operations Command, are launched from bases in nearby Djibouti, and from a secret CIA base in the Persian Gulf, which The Associated Press has agreed not to identify at the request of U.S. officials.

Other drones are launched from Ethiopia and a temporary base in the Seychelles.

The first drones were built in the early 1990s, mostly for surveillance missions, employed in the war zones of Bosnia and Kosovo.

The first known lethal drone strike was in Yemen, almost a decade ago, when a CIA drone armed with Hellfire missiles opened fire on convicted terror suspect Kamal Derwish of the Lackawanna Six, who had fled to Yemen.

The Lackawanna Six were Yemeni-American childhood friends who were convicted of providing support to al-Qaida, after attending an al-Qaida training camp in Afghanistan in 2001.