WASHINGTON — Osama bin Laden, the glowering mastermind behind the Sept. 11, 2001, terror attacks that murdered thousands of Americans, was killed in an operation led by the United States, President Barack Obama said Sunday.

"Justice has been done," said the president in a dramatic late-night announcement at the White House.

A small team of Americans killed bin Laden in a firefight Sunday at a compound in Pakistan, the president said, and took custody of his remains. Americaj officials said they were being handled in accordance with Islamic tradition.

A jubilant crowd gathered outside the White House as word spread of bin Laden's death after a global manhunt that lasted nearly a decade.

Former President George W. Bush, who was in office on the day of the attacks, issued a written statement hailing bin Laden's death as a momentous achievement. "The fight against terror goes on, but tonight America has sent an unmistakable message: No matter how long it takes, justice will be done," he said.

Obama said he ordered the operation after receiving undisclosed intelligence information. Senior administration officials said the terrorist mastermind was found inside a custom-built compound with two security gates. They said it appeared to hvae been constructed to harbor one high-value target and that for undisclosed reasons, officials became clear the hideout was bin Laden's.

Officials also said they believe the death puts al-Qaida on a path of decline that will be difficult to reverse, but there was no word on the whereabouts of bin Laden's second-in-command, Ayman al-Zawahri.

The stunning end to the world's most widely-watched manhunt came just months before the 10th anniversary of the Sept. 11 attacks on the World Trade Centers and Pentagon, orchestrated by bin Laden's al-Qaida organization, that killed more than 3,000 people.

The attacks a decade ago seemed to come out of nowhere, even though al-Qaida had previously damaged American targets overseas.

The terrorists hijacked planes, flew one of them into one of Manhattan's Twin Towers — and, moments later, into the other one. Both buildings collapsed, trapping thousands inside and claiming the lives of firefighters and others who had rushed to help them.

A third plane slammed into the Pentagon, defacing the symbol of America's military night. A fourth crashed in rural Pennsylvania after passengers overpowered the hijackers and forced the craft from the air — before it could hit its intended target in Washington.

The attacks set off a chain of events that led the United States into wars in Afghanistan, and then Iraq, and America's entire intelligence apparatus was overhauled to counter the threat of more terror attacks at home.

A senior administration official says Obama gave the final order for U.S. officials to go after bin Laden on Friday. The official added that a small team found their quarry hiding in a large home in an affluent suburb of Islamabad. The raid occurred in the early morning hours Sunday.

Administration officials offered some details of the operation.

Based on statements given by U.S. detainees, intelligence officials have known for years that bin Laden trusted one al-Qaida courier in particular and they believed he might be living with him in hiding. In November, intelligence officials found out where he was living, a huge fortified compound in an affluent suburb of Islamabad. It was surrounded by walls as high as 18 feet high, topped with barbed wire. There were two security gates and no phone or Internet running into the house.

Intelligence officials believed the $1 million home was custom-built to harbor a major terrorist. CIA experts analyzed whether it could be anyone else, but time and again, they decided it was almost certainly bin Laden.

Three adult males were also killed in Sunday's raid, including one of bin Laden's sons, whom officials did not name. One of bin Laden's sons, Hamza, is a senior member of al-Qaida.

Obama spoke with Bush and former President Bill Clinton Sunday night to inform them of the developments.

Obama struck a less than boastful tone in his brief announcement, although he said the death of bin Laden was "the most significant achievement to date in our nation's effort to defeat al-Qaida.

"His death does not mark the end of our effort. There's no doubt that al-Qaida will continue to pursue attacks against us. We must and we will remain vigilant," he added.

Moments after he spoke, American officials cautioned that the events could lead to heightened threats against the United States.

Officials said the U.S. would ensure that bin Laden's body was handled in accordance with Islamic tradition.