Step by step: How the U.S. killed bin Laden

Soon after coming to office, President Obama made it clear that killing or capturing Osama bin Laden was one of his highest national-security priorities.

But it wasn't until last summer that intelligence officials caught a break in their pursuit of al-Qaida's leader.

Numerous behind-the-scenes accounts of the killing of bin Laden have been flowing since Sunday night's stunning announcement of his death. Here is what National Journal knows now:

In September 2010, the CIA presented Obama with a set of assessments that indicated bin Laden could be hiding in a compound in northwest Pakistan. Starting in mid-March, the president convened at least nine National Security Council meetings to discuss the intelligence suggesting that bin Laden was possibly hiding out virtually in plain sight.

The CIA developed its theory of bin Laden being there through leads gained from individuals in al-Qaida's inner circle and from fighters captured after September 11. Intelligence officials were repeatedly told about one courier working for bin Laden as someone that America's most-wanted man deeply trusted.

The detainees provided the courier's nickname to U.S. officials and identified him as a protégé of Khalid Sheikh Mohammed and a trusted assistant of Abu Faraj al-Libbi, once al-Qaida's third-highest-ranking official (he was captured in 2005). Intelligence officials identified the courier's name four years ago, but it took two more years to zero in on areas of Pakistan where he and his brother operated.

Finally in August 2010, they were able to track him down to the sprawling compound in Abbottabad, a relatively affluent enclave favored by retired Pakistani military, a senior administration official said. The compound was massive and tightly guarded with only two points of access which raised American suspicions that it was more than some Pakistani General's retirement home.

"The physical security measures of the compound are extraordinary," a senior administration official said. "It has 12- to 18-foot walls topped with barbed wire.... Internal walls sectioned off different portions of the compound to provide extra privacy.... The residents of the compound burn their trash, unlike their neighbors, who put the trash out for collection."

Bin Laden and his family were living on the top two floors of the three-story structure, the most prominent building on the property. The compound had neither Internet nor telephone access which was unusual, to say the least, since the property was estimated by American officials to be worth approximately $1 million.

"Many of his terrorist associates in other parts of Pakistan and throughout the region are living in much more dire conditions," a senior administration official said. "You have to be wondering what they're thinking at this moment when they see that their leader was living, relatively speaking, high on the hog." In the hours since bin Laden's killing, American officials have clearly pointed to his luxurious quarters to dispirit al-Qaida forces and Taliban sympathizers who might lament their woeful conditions on the battlefield while their leader, rather than roughing it in some cave, as many had long believed, lived like a wealthy Pakistani mansion.

Intelligence analysts studied the compound, which was built in 2005, carefully. While convinced that a high-value target was staying there, they assessed whether it could be someone other than bin Laden, perhaps his second-in-command, Ayman al-Zawahiri.

"We conducted red-team exercises and other forms of alternative analysis to check our work," the senior administration official said. "No other candidate fit the bill as well as bin Laden did."

The president finally gave the order on Friday morning for the operation to pursue bin Laden-just before he departed for Alabama to visit areas ravaged by last week's tornadoes, a senior administration official said.

Early Sunday morning in Pakistan, the strike began.

The team that killed Osama bin Laden would have taken him alive if it had the chance, said John Brennan, a counterrorism adivser to Obama, but he said administration officials were doubtful that bin Laden would go down without a fight.

"If we had the opportunity to take him alive, we would have done that," said Brennan, adding that the president "put a premium on making sure that our personnel were protected" during the raid.

By 1 p.m. in Washington on Sunday, top advisers had gathered at the White House. Around 2 p.m., shortly after returning to the White House after playing nine holes of golf, Obama huddled with them to review final preparations for the operation. He returned to the Situation Room at 3:32 p.m. for another update, and by 3:50 p.m. he was given word that bin Laden was "tentatively identified" as among those killed in the operation.

More information trickled in indicating that it was indeed bin Laden-including preliminary DNA analysis, Brennan said. At 7 p.m., Obama was told it was a "high probability" that it was, indeed, bin Laden.

"We got him," Obama declared, according to Brennan, after he was confident that U.S. forces captured the terrorist.

The entire operation took just 40 minutes and involved a small U.S. team of commandos, a senior administration official said.

The detachment of Navy SEALs earmarked for the mission spent most of April practicing the assault at a secret base inside neighboring Afghanistan, according to a senior military official with direct knowledge of their preparations. The SEALs constructed a full-scale mock-up of the compound to practice the most effective ways of breaching its perimeter and assaulting various numbers of defenders, the official said. During the weeks of preparations, the compound in Pakistan was kept under round-the-clock surveillance by the U.S., but the official declined to say what combination of drones, aircraft, or satellites was used to monitor the target site. The SEALs, meanwhile, were kept in a constant state of readiness in case the order came to launch the mission sooner than expected because of signs that bin Laden was preparing to depart the fortified building, the official said.

As the SEALS conducted the raid, Obama and senior advisers monitored the operation in real time, Brennan told reporters on Monday afternoon. In the Situation Room, the mood was tense.

"It was probably one of the most anxiety-filled periods of time in the lives of the people who were assembled here yesterday," Brennan said. "The minutes passed like days and the president was very concerned about the security of our personnel."

A stealth raid wasn't the only option Obama considered. One administration official said that Obama advisers presented him with the option of bombing the compound. Advisers also presented the president with plans to conduct raids on the compound on two other occasions, but Obama rejected the plan, according to another administration official.

In addition to bin Laden, four others were killed in the strike on the compound: an adult son of bin Laden, two of bin Laden's couriers, and a woman, who an administration official said was used as a human shield.

Administration officials offered scant details about how bin Laden conducted himself in his final moments, only saying that he was felled in a firefight and suffered a wound to the left side of his face.

It's also unclear how long bin Laden had been living at the compound, but intelligence officials say multiple sources indicated that he was there with his youngest wife and several family members. Brennan said the woman who died in the raid was believed one of bin Laden's wives, and he used her as a human shield.

No Americans were killed in the operation, according to U.S. officials, which was kept secret from the Pakistani government until after it was completed. But a U.S. helicopter was lost in the operation because of mechanical failure, creating white knuckle moments in the Situation Room.

"When that helicopter was seen to be unable to move, all of sudden you had to go into Plan B," Brennan said. "And they did it flawlessly ... but seeing that helicopter in a place and in condition that it was supposed to be, I think for me, and for other people in the room, was" tense.

Later in the evening, Obama called former presidents Bill Clinton and George W. Bush to deliver the news.

When the president walked into the East Room to deliver the news to the world late Sunday night, more than a dozen White House staffers-including Vice President Joe Biden, Chief of Staff William Daley, Secretary of State Hillary Clinton, CIA Director Leon Panetta, and National Security Adviser Tom Donilon-were on hand to hear him make the announcement in person.

Hundreds of people, meanwhile, had gathered outside the White House to celebrate the news.

By Monday, bin Laden was buried in the North Arabian Sea. A U.S. military officer read the Islamic funeral rites over bin Laden in English and a native speaker translated his words into Arabic before he was interred.

While White House officials said that the killing of bin Laden is a huge milestone in the battle against al-Qaida, the battle ahead remains difficult.

"This does not mean we are putting down our guard as far as al-Qaida is concerned," Brennan said. "It may be a mortally wounded tiger that still has some life in it, and it's dangerous. We need to keep up the pressure."

Marc Ambinder and Rebecca Kaplan contributed.