The unmanned planes, impervious to radar detection, were able to fly in Pakistani airspace without Pakistan's knowledge, unlike the Predator and Reaper drones that attack militants near the Afghan frontier, a former official who spoke on the condition of anonymity told The Washington Post.
"It's not like you can just park a Predator overhead -- the Pakistanis would know," the former official said.
The surveillance also involved satellites, eavesdropping gear and CIA operatives based at a safe house in Abbottabad. The CIA would not comment on its methods.
The secret incursions are another sign of U.S. distrust of Pakistan.
The intelligence chief, Lt. Gen. Ahmed Shuja Pasha, last week offered to quit over his failure to detect or prevent the May 2 U.S. raid, which he denounced as a "breach of Pakistan's sovereignty."
The U.S. Air Force acknowledged the existence of a stealth drone, Lockheed Martin's RQ-170 Sentinel, in 2009, two years after it was spotted at an airfield in Kandahar, Afghanistan. It has the distinctive, bat-winged shape of larger stealth warplanes.
The stealth drones also were used during the raid to send images to Washington and monitor the Pakistani response.
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