Al Qaeda leader Osama bin Laden, a mastermind of the largest terrorist attack in American history, was killed Sunday in Pakistan in a military operation after the U.S. learned of his location.

The death capped a manhunt of a decade for the architect of the Sept. 11, 2001, attacks that left nearly 3,000 people dead and dramatically altered U.S. foreign policy and the nation’s sense of security.

“Justice has been done,” President Barack Obama said from the White House, where he made the dramatic announcement late Sunday. “The United States has conducted an operation that killed Osama bin Laden, the leader of al Qaeda and a terrorist who is responsible for the murder of thousands of innocent men, women and children.”

He said that he had ordered the attack earlier in the day after it became clear last week that there was sufficient intelligence, following a lead about his whereabouts that first surfaced in August. Mr. bin Laden was killed with a “head shot” during the firefight, a senior administration official said. The raid was conducted by U.S. military personnel operating under the authority of the Central Intelligence Agency.

Mr. Bin Laden’s body was buried at sea, in order to be in accordance with Islamic tradition that burial take place within 24 hours, according to a person familiar with the situation. The Saudis declined a U.S. offer to take the body, this person said.

Hundreds of onlookers gathered outside the White House—and at Ground Zero in New York City, where the twin towers of the World Trade Center once stood—to mark the moment. Frank Cantwell, a 64-year-old operating engineer at the construction site there, said there were only a few people present when the news first broke. “It’s long overdue,” he said. “You can sort of hear the silent cheers of 3,000 ghosts.”

The elusive al Qaeda leader was killed in a targeted assault in the Pakistani city of Abbottabad, roughly 40 miles outside the capital city of Islamabad. The raid was conducted by a small helicopter-borne strike team, a senior U.S. administration official said. The team was on the compound for under 40 minutes, the official said.

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