The United States had been trying to kill Osama bin Laden for 13 years, since the administration of President Bill Clinton.
"Bin Laden has been our national enemy self declared for far longer than any one person in our history," said Richard Clarke, an ABC News contributor who served as a counterterrorism advisor to Clinton and both presidents Bush.
For years the trail had gone cold, some thought he had left the region and Pakistani officials even claimed bin Laden must be dead. In the end he was found in a house where he may have been living for as many as six years, almost next door to Pakistani military installations and less than 100 miles from the Pakistani capital of Islamabad.
The son of a prominent and wealthy Saudi family, bin Laden first went to Afghanistan to fight the Russians but then turned on the United States.
On his orders thousands died, in the 1998 attacks on two U.S. embassies in Africa, in the attack on the USS Cole in Yemen in 2000, and the 9/11 attacks of which he seemed so proud. Bin Laden boasted on tape that "We calculated in advance the number of casualties."
The discovery of bin Laden's hiding place in a million-dollar mansion in Pakistan was the result of a masterful CIA intelligence operation that focused on the courier who was his connection to the outside world.
"We had to piece it together," explained John Brennan, counterterrorism advisor to President Obama, "get [the courier's] nom de guerre, associate it with a real name and track it until we got to the compound."
Every video or audio message recorded by bin Laden went by courier, so each new message became an opportunity for the CIA to find him.
"That was his Achilles heel," said John Nagl, president of the Center for a New American Security and former lieutenant colonel in the U.S. Army. "We were able to use the information provided by tracking his couriers to hunt him down and bring him to justice."
U.S. officials say detainees held at Guantanamo helped lead them to the courier.
In one early clue, according to a secret Pentagon cable made public by Wikileaks, senior Al Qaeda commander Abu al Libi told interrogators he became "the official messenger" for bin Laden and for a year in 2003 "moved his family to Abbottabad, Pakistan" -- the city near Islamabad where bin Laden was killed Sunday.
By August of last year, the CIA had a sharp focus on a newly built compound in the Abbottabad, suspicious of its large size and extensive security features, including a seven-foot wall on the terrace so the very tall bin Laden could be outside without being seen.
A family of the same size as bin Laden's was seen here, although bin Laden himself was never actually spotted.
But since August he had sent out, at least two new taped messages. One of them, remarkably, last October, focused on the severe flooding that had just occurred in the Abbottabad region.