A U.S. official says Osama bin Laden went down firing at the Navy SEALs who stormed his compound.
An official familiar with the operation says bin Laden was hit by a barrage of carefully aimed return fire.
The official spoke on condition of anonymity because aspects of the operation remain classified.
The official says two dozen SEALs in night-vision goggles dropped into the high-walled compound in Pakistan by sliding down ropes from Chinook helicopters in the overnight raid.
U.S. officials say bin Laden was killed near the end of the 40-minute raid.
The SEALs retrieved bin Laden's body and turned the remaining detainees over to Pakistani authorities.
Long believed to be hiding in caves, bin Laden was tracked down in a costly, custom-built hideout not far from a Pakistani military academy. The stunning news of his death prompted relief and euphoria outside the White House and around the globe, yet also deepening fears of terrorist reprisals against the United States and its allies.
"Justice has been done," President Barack Obama said late Sunday from the White House in an announcement that seemed sure to lift his own political standing.
The military operation took mere minutes, and there were no U.S. casualties.
U.S. Blackhawk helicopters ferried about two dozen troops from Navy SEAL Team Six, a top military counter-terrorism unit, into the compound identified by the CIA as bin Laden's hideout -- and back out again in less than 40 minutes. Bin Laden was shot in the head, officials said, after he and his bodyguards resisted the assault.
Three adult males were also killed in the 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. U.S. officials also said one woman was killed when she was used as a shield by a male combatant, and two other women were injured.
The U.S. official who disclosed the burial at sea said it would have been difficult to find a country willing to accept the remains. Obama said the remains had been handled in accordance with Islamic custom, which requires speedy burial.
"I heard a thundering sound, followed by heavy firing. Then firing suddenly stopped. Then more thundering, then a big blast," said Mohammad Haroon Rasheed, a resident of Abbottabad, Pakistan, after the choppers had swooped in and then out again.
Bin Laden's death marks a psychological triumph in a long struggle that began well before the Sept. 11 attacks. Al-Qaida was also blamed for the 1998 bombings of two U.S. embassies in Africa that killed 231 people and the 2000 attack on the U.S.S Cole that killed 17 American sailors in Yemen, as well as countless other plots, some successful and some foiled.
"We have rid the world of the most infamous terrorist of our time," CIA director Leon Panetta declared to employees of the agency in a memo Monday morning.
He warned that "terrorists almost certainly will attempt to avenge" the killing of a man deemed uncatchable. "Bin Laden is dead. Al-Qaida is not," Panetta said.
Retaliatory attacks against the U.S. and Western targets could come from members of al-Qaida's core branch in the tribal areas of Pakistan, al-Qaida franchises in other countries, and radicalized individuals in the U.S. with al-Qaida sympathies, according to a Homeland Security Department intelligence alert issued Sunday and obtained by The Associated Press.
While the intelligence community does not have insight into current al-Qaida plotting, the department believes symbolic, economic and transportation targets could be at risk, and small arms attacks against other targets can't be ruled out.
In all, nearly 3,000 were killed in the Sept. 11 attacks nearly 10 years ago, the worst terror assault on American soil.
As news of bin Laden's death spread, hundreds of people cheered and waved American flags at ground zero in New York, the site where al-Qaida hijacked jets toppled the twin towers of the World Trade Center. Thousands celebrated all night outside the White House gates.
As dawn came the crowd had thinned yet some still flowed in to be a part of it. A couple of people posed for photographs in front of the White House while holding up front pages of Monday's newspapers announcing bin Laden's death.
"It's a moment people have been waiting for," said, Eric Sauter, 22, a University of Delaware student who drove to Washington after seeing TV coverage of the celebrations.
The development seems certain to give Obama a political lift as the nation swelled in pride. Even Republican critics lauded him.
But its ultimate impact on al-Qaida is less clear.
The greatest terrorist threat to the U.S. is now considered to be the al-Qaida franchise in Yemen, far from al-Qaida's core in Pakistan. The Yemen branch almost took down a U.S.-bound airliner on Christmas 2009 and nearly detonated explosives aboard two U.S. cargo planes last fall. Those operations were carried out without any direct involvement from bin Laden.
The few fiery minutes in Abbottabad followed years in which U.S. officials struggled to piece together clues that ultimately led to bin Laden, according to an account provided by senior administration officials who spoke on condition of anonymity because of the sensitivity of the operation.
Based on statements given by U.S. detainees since the 9/11 attacks, they said, intelligence officials have long known that bin Laden trusted one al-Qaida courier in particular, and they believed he might be living with him in hiding.
Four years ago, the United States learned the man's identity, which officials did not disclose, and then about two years later, they identified areas of Pakistan where he operated. Last August, the man's residence was found, officials said.
"Intelligence analysis concluded that this compound was custom built in 2005 to hide someone of significance," with walls as high as 18 feet and topped by barbed wire, according to one official. Despite the compound's estimated $1 million cost and two security gates, it had no phone or Internet running into the house.
By mid-February, intelligence from multiple sources was clear enough that Obama wanted to "pursue an aggressive course of action," a senior administration official said. Over the next two and a half months, the president led five meetings of the National Security Council focused solely on whether bin Laden was in that compound and, if so, how to get him, the official said.
Obama made a decision to launch the operation on Friday, shortly before flying to Alabama to inspect tornado damage, and aides set to work on the details.
The president spent part of his Sunday on the golf course, but cut his round short to return to the White House for a meeting where he and top national security aides reviewed final preparations for the raid.
Two hours later, Obama was told that bin Laden had been tentatively identified.
Bin Laden was positively identified through "multiple methods," a senior Pentagon official said Monday, adding that he had personally seen a photo of the corpse. The official, who spoke on condition of anonymity because he was not authorized to speak on the record, declined to say what other methods were used.
The remains were taken to a U.S. warship, but the official declined to say which one and where the ship was.
Panetta was directly in charge of the military team during the operation, according to one official, and when he and his aides received word at agency headquarters that bin Laden had been killed, cheers broke out around the conference room table.
Administration aides said the operation was so secretive that no foreign officials were informed in advance, and only a small circle inside the U.S. government was aware of what was unfolding half a world away.
In his announcement, Obama said he had called Pakistani President Asif Ali Zardari after the raid, and said it was "important to note that our counter-terrorism cooperation with Pakistan helped lead us to bin Laden and the compound where he was hiding."
One senior administration told reporters, though, "we were very concerned ... that he was inside Pakistan, but this is something we're going to continue to work with the Pakistani government on."
The compound is about a half-mile from a Pakistani military academy, in a city that is home to three army regiments and thousands of military personnel. Abbottabad is surrounded by hills and with mountains in the distance.
Critics have long accused elements of Pakistan's security establishment of protecting bin Laden, though Islamabad has always denied it, and in a statement the foreign ministry said his death showed the country's resolve in the battle against terrorism.
Still, bin Laden's location raised pointed questions of whether Pakistani authorities knew the whereabouts of the world's most wanted man.
Whatever the global repercussions, bin Laden's death marked the end to a manhunt that consumed most of a decade that began in the grim hours after bin Laden's hijackers flew planes into the World Trade Center twin towers in Manhattan and the Pentagon across the Potomac River from Washington. A fourth plane was commandeered by passengers who overcame the hijackers and forced the plane to crash in the Pennsylvania countryside.