Extensive surveillance of Osama bin Laden's hideout from a nearby CIA safe house in Abbottabad led to his killing in a Navy SEALs operation, U.S. officials said, a revelation likely to further embarrass Pakistan's spy agency and strain ties.
The U.S. officials, quoted by the Washington Post on Friday, said the safe house was the base for an intelligence-gathering operation that began after Mr. bin Laden's compound was discovered last August, and which was so exhaustive that the CIA asked Congress to reallocate tens of millions of dollars to fund it.
"The CIA's job was to find and fix," the Post quoted one U.S. official as saying. "The intelligence work was as complete as it was going to be, and it was the military's turn to finish the target."
U.S. officials told the New York Times that intelligence gathered from computer files and documents seized at his compound showed Mr. bin Laden had for years orchestrated al-Qaeda attacks from the Pakistani town, and may have been planning a strike on the U.S. rail sector this year, the 10th anniversary of the Sept. 11, 2001 attacks.
One U.S. official said there was no indication from the intelligence that further plans were drawn up for the railway plot or that steps were taken to carry it out. The U.S. Department of Homeland Security said it had no information of an imminent threat.
The fact that Mr. bin Laden was found in a garrison town - his compound was a stone's throw away from a major military academy - has embarrassed Pakistan and the covert raid by U.S. commandos that led to his killing has angered its military.
On Thursday, the Pakistan army threatened to halt counter-terrorism co-operation with the United States, if it conducted another, similar unilateral strike.
A major Islamist party in Pakistan, Jamaat-e-Islami, called for mass protests on Friday against what it called a violation of sovereignty by the U.S. raid. It also urged the government to end support for U.S. battles against militants.
A senior Pakistani security official also charged that U.S. troops had killed the unarmed al-Qaeda leader in "cold blood".
The criticism from Pakistan is likely to fray a relationship that Washington deems vital to defeating the al-Qaeda movement that Mr. bin Laden led and winning its war in neighbouring Afghanistan.
A U.S. acknowledgment that Mr. bin Laden was unarmed when shot in the head - as well as the sea burial of his body, a rare practice in Islam - have also drawn criticism in the Arab world and Europe, where some have warned of a backlash.
Few Americans appear to have any qualms about how Mr. bin Laden was killed, and on Thursday, scores of people cheered President Barack Obama during a visit to New York's Ground Zero, site of the twin towers al-Qaeda levelled on Sept. 11, 2001, to comfort a city still scarred by attacks that killed nearly 3,000 people.
Mr. Obama said the killing of Mr. bin Laden "sent a message around the world, but also sent a message here back home, that when we say we will never forget, we mean what we say."
Friction between Washington and Pakistan has focused on the role of Pakistan's top security service, the ISI or Inter-Services Intelligence Directorate.
Foreign Secretary Salman Bashir denied Pakistani forces or the ISI aided al-Qaeda. "The critique of the ISI is not only unwarranted, it cannot be validated," he said.
Lobbyists for Pakistan in Washington have launched an intense campaign on Capitol Hill to counter accusations that Islamabad deliberately gave refuge to Mr. bin Laden.
But many Americans are questioning how the al-Qaeda leader could live for years in a Pakistani town teeming with military personnel, 50 kilometres from the capital, Islamabad. Two U.S. lawmakers have also complained about the billions in U.S. civilian and military aid to impoverished Pakistan.
Seeking to repair ties, U.S. Secretary of State Hillary Clinton said in Rome on Thursday that Washington was still anxious to maintain its alliance with Islamabad.
The Pakistani army and spy agency have supplied intelligence to the United States, arrested al Qaeda figures and taken on militants in areas bordering Afghanistan.
"It is not always an easy relationship," Ms. Clinton said. "But, on the other hand, it is a productive one for both our countries and we are going to continue to co-operate between our governments, our militaries, our law-enforcement agencies."
But Pakistan's army, facing rare criticism at home over the U.S. operation, warned the United States it would risk this co-operation if it conducted another assault.
Chief of Staff General Ashfaq Kayani "made it clear that any similar action violating the sovereignty of Pakistan will warrant a review on the level of military/intelligence co-operation with the United States", the army said.
It was unclear if such attacks included drone strikes which the U.S. military regularly conducts against militants along Pakistan's border with Afghanistan. Pakistan has denied harbouring any members of al-Qaeda.
The army also said it would conduct an investigation into failures by its intelligence to detect the world's most wanted man in its own backyard.
The CIA had spent several months monitoring Mr. bin Laden's hideout, watching and photographing residents and visitors from a rented house nearby, according to U.S. officials quoted in the New York Times and Washington Post.
Observing from behind mirrored glass, CIA officers used cameras with telephoto lenses and infrared imaging equipment to study the compound, and they used sensitive eavesdropping equipment to try to pick up voices from inside the house and to intercept cellphone calls, the New York Times said. A satellite used radar to search for possible escape tunnels.
The U.S. administration has refused to be drawn on details on the raid, but, in a further sign of fractious relations between the allies, senior Pakistani security officials told Reuters that U.S. accounts had been misleading.
In Washington, people familiar with the latest U.S. government reporting on the raid told Reuters on Thursday that only one of four principal targets shot to death by U.S. commandos was involved in any hostile fire.
As the elite Navy SEALs moved in on a guest house inside Mr. bin Laden's compound, they were met with fire and shot a man in the guest house. He proved to be Abu Ahmed al-Kuwaiti, an al-Qaeda courier U.S. intelligence agencies had long been tracking.
The commandos then entered the main residence, where they killed another courier and a son of Mr. bin Laden, the sources said. They finally shot and killed the al-Qaeda leader in a top-floor room after having earlier fired at him as he poked his head out of a door or over a balcony.
U.S. officials originally spoke of a 40-minute firefight. The White House has blamed the "fog of war" for the changing accounts.
Mr. Obama visited New York to say he had made good on a 10-year-old promise by his predecessor, George W. Bush, who declared at the smouldering wreckage of the World Trade Center three days after the Sept. 11 attacks, "The people who knocked these buildings down will hear all of us soon."
"We have been waiting for this for 10 years. It puts a little more American pride in people," said Al Fiammetta, 57, a safety engineer who said he had cleared debris at Ground Zero.
Mr. Obama signalled in an interview with the CBS television program 60 Minutes that Mr. bin Laden's death confirmed his commitment to begin drawing down troops in Afghanistan in July. "We don't need to have a perpetual footprint of the size that we have now," he said in a published excerpt.