The CIA employed sophisticated new stealth drone aircraft to fly dozens of secret missions deep into Pakistani airspace and monitor the compound where Osama bin Laden was killed, current and former US officials said.
Using unmanned planes designed to evade radar detection and operate at high altitudes, the agency conducted clandestine flights over the compound for months before the May 2 assault in an effort to capture high-resolution video that satellites could not provide.
The aircraft allowed the CIA to glide undetected beyond the boundaries that Pakistan has long imposed on other US drones, including the Predators and Reapers that routinely carry out strikes against militants near the border with Afghanistan.
The agency turned to the new stealth aircraft "because they needed to see more about what was going on" than other surveillance platforms allowed, said a former US official familiar with the details of the operation.
"It's not like you can just park a Predator overhead - the Pakistanis would know," added the former official, who, like others interviewed, spoke on the condition of anonymity, citing the sensitivity of the program.
The monitoring effort also involved satellites, eavesdropping equipment and CIA operatives based at a safe house in Abbottabad, the city where bin Laden was found. The agency declined to comment for this article.
The CIA's repeated secret incursions into Pakistan's airspace underscore the level of distrust between the United States and a country often described as a key counterterrorism ally, and one that has received billions of dollars in US aid.
Pakistan's spy chief, Lt Gen Ahmed Shuja Pasha, last week offered to resign over the government's failures to detect or prevent a US operation that he described as a "breach of Pakistan's sovereignty."
The country's military and main intelligence service have come under harsh criticism since the revelation that bin Laden had been living in a garrison city - in the midst of the nation's military elite - possibly for years.
The new drones represent a major advance in the capabilities of remotely piloted planes, which have been the signature American weapon against terrorist groups since the attacks of Sept 11, 2001.
In 2009, the Air Force acknowledged the existence of a stealth drone, a Lockheed Martin model known as the RQ-170 Sentinel, two years after it was spotted at an airfield in Kandahar, Afghanistan. The aircraft bears the distinct, bat-winged shape of larger stealth warplanes. The operational use of the drones has never been described by official sources.
The extensive aerial surveillance after the compound was identified in August helps explain why the CIA went to Congress late last year, seeking permission to transfer tens of millions of dollars within agency budgets to fund intelligence-gathering efforts focused on the complex.
The stealth drones were used on the night of the raid, providing imagery that President Barack Obama and members of his national security team appear in photographs to have been watching as US Navy Seals descended on the compound shortly after 1 am in Pakistan. The drones are also equipped to eavesdrop on electronic transmissions, enabling US officials to monitor the Pakistani response.
The use of one of the aircraft on the night of the raid was reported by the National Journal's Marc Ambinder, who said in a tweet May 2 that an "RQ-170 drone [was] overhead."
The CIA never obtained a photograph of bin Laden at the compound or other direct confirmation of his presence before the assault, but the agency concluded after months of watching the complex that the figure frequently seen pacing back and forth was probably the al Qaeda chief.
The operation in Abbottabad involved another US aircraft with stealth features, a Black Hawk helicopter equipped with special cladding to dampen noise and evade detection during the 90-minute flight from a base in Afghanistan. The helicopter was intentionally destroyed by US forces - leaving only a tail section intact - after a crash landing at the outset of the raid.
The assault and the months of surveillance leading up to it involved venturing into some of Pakistan's most sensitive terrain. Because of the compound's location - near military and nuclear facilities - it was surrounded by Pakistani radar and other systems that could have detected encroachment by Predators or other non-stealth surveillance planes, according to US officials.
"It's a difficult challenge trying to secure information about any area or object of interest that is in a location where access is denied," said retired Air Force Lt Gen David Deptula, who served as head of intelligence and surveillance for that service.
The challenge is multiplied, he said, when the surveillance needs to be continuous, which "makes non-stealthy slow-speed aircraft easier to detect."
Satellites can typically provide snapshots of fixed locations every 90 minutes. "Geosynchronous" satellites can keep pace with the Earth's rotation and train their lenses on a fixed site, but they orbit at 22,500 miles up. By contrast, drones fly at altitudes between 15,000 and 50,000 feet.
In a fact sheet released by the Air Force, the RQ-170 is described as a "low observable unmanned aircraft system," meaning that it was designed to hide the signatures that make ordinary aircraft detectable by radar and other means. The sheet provides no other technical details.
Stealth aircraft typically use a range of radar-defeating technologies. Their undersides are covered with materials designed to absorb sound waves rather than bouncing them back at sensors on the ground. Their engines are shielded and their exhaust diverted upward to avoid heat trails visible to infrared sensors.
The Air Force has not explained why the RQ-170 was deployed to Afghanistan, where US forces are battling insurgents with no air defenses. Air Force officials declined to comment for this story.
Over the past two years, the US military has provided many of its Afghanistan-based Predators and Reapers to the CIA for operations in Pakistan's tribal region, where insurgent groups are based.
The stealth drones followed a similar path across the Pakistan border, officials said, but then diverged and continued toward the compound in Abbottabad.
US officials said the drones wouldn't have needed to be directly over the target to capture high-resolution video, because they are equipped with cameras that can gaze at steep angles in all directions.
"It's all geometry and slant ranges," said a former senior defense intelligence official.
Still, the missions were regarded as particularly risky because, if detected, they might have called Pakistani attention to US interest in the bin Laden compound.
"Bin Laden was in the heart of Pakistan and very near several of the nuclear weapons production sites," including two prominent complexes southeast of Islamabad, said David Albright, a nuclear weapons proliferation expert at the Institute for Science and International Security.
To protect such sites, Pakistan's military has invested heavily in sophisticated radar and other aircraft-detection systems. "They have traditionally worried most about penetration from India, but also the United States," Albright said.
Largely because of those concerns, Pakistan has placed strict limits on the number and range of CIA-operated Predators patrolling the country's tribal areas.US officials refer to the restricted zones as "flight boxes" that encompass North and South Waziristan.