Osama bin Laden, mastermind of the Sept. 11, 2001, terrorist attacks, has been killed by U.S. forces in what is being described as a surgical strike at a compound in northern Pakistan, ending one of the longest and costliest manhunts in history.

President Obama announced the news late Sunday at the White House, calling the death of bin Laden “the most significant achievement to date” in the war against al-Qaida — — a battle that has led the U.S. into protracted and bloody conflicts in both Afghanistan and Iraq.

“Justice has been done,” Obama said.

The news released a decade’s worth of emotion as Americans, cheering, waving flags and singing the national anthem, streamed to the site of the World Trade Center in New York City, the gates of the White House in Washington, D.C., and across the nation.

Obama said U.S. intelligence tracked the terrorist leader to a redoubt near the Pakistani town of Abbottabad. His movements were monitored for months until a small team of U.S. operatives moved on the compound early Monday local time. An American official said bin Laden had been buried at sea in accordance with Islamic tradition that calls for a speedy interment of the body.

“After a firefight, they killed Osama bin Laden and took custody of his body,” the president said, warning that the U.S. must remain vigilant because al-Qaida will “continue to pursue attacks against us.”

Even before the Sept. 11 attacks that killed nearly 3,000 people and became a defining moment in U.S. history, American officials had bin Laden in their sights. But he managed to elude capture, moving to Sudan and later to Afghanistan, where he was sheltered by the Taliban even after the regime was toppled by a U.S. invasion. Bin Laden is thought to have continued a secret nomadic existence, moving in and around the mountainous border region that straddles Afghanistan and Pakistan.

Obama said that almost two years ago, he formally ordered the CIA to make finding bin Laden a top priority. The administration said years of intelligence gathering began to pay off in August, when authorities discovered a heavily fortified compound outside Islamabad that appeared to be custom-built for harboring someone as notorious — and resourceful — as bin Laden.

“It was far from certain, and it took many months to run this thread to ground,” Obama said in his address.

“I met repeatedly with my national security team as we developed more information about the possibility that we had located bin Laden hiding within a compound deep inside of Pakistan,” he said. “Finally, last week, I determined that we had enough intelligence to take action, and authorized an operation to get Osama bin Laden and bring him to justice.”

The order was given Friday morning, U.S. officials said, shortly before the president left to tour tornado-raked areas in Alabama. Officials said the final operation — which was under the direction of CIA head Leon Panetta — was so secret that no foreign officials were informed and only a small circle in Washington was aware.

According to U.S. officials who spoke to The Associated Press, four helicopters swooped in early Monday and killed bin Laden in a raid on the fortress-like compound in Abbottabad. The town north of the capital, Islamabad, is home to three army regiments and the Kakul Military Academy, an army officer training center. The location raised pointed questions of whether Pakistani authorities knew the whereabouts of the world’s most wanted man.

Pakistani officials said a son of bin Laden and three other people were killed.

The location was far from the remote mountain caves along the Pakistan-Afghanistan tribal border where most intelligence assessments had put bin Laden in recent years.

An American administration official said the compound was built in 2005 at the end of a narrow dirt road with “extraordinary” security measures. He said it had 12- to 18-foot walls topped with barbed wire with two security gates, and no telephone or Internet service.

A Pakistan intelligence official said the property where bin Laden was staying was 3,000 square feet.

One Pakistani official said the choppers took off from a Pakistani air base, suggesting some cooperation in the raid. Obama said Pakistan had provided some information leading to the raid.

Critics have long accused elements of Pakistan’s security establishment of protecting bin Laden, though Islamabad has always denied this. Ties between the U.S. and Pakistan have reached a low point in recent months over the future of Afghanistan, and any hint of possible Pakistani collusion with bin Laden could have major reverberations.

Pakistan’s foreign ministry said the death of bin Laden shows the resolve of that country and the world to battle terrorism — a resolve that has been frequently questioned by U.S. officials over the past decade.

Pakistan’s first official statement about the operation acknowledged that the raid was a U.S. operation but did not elaborate.

Obama telephoned his Pakistani counterpart, Asif Ali Zardari, after the raid, and the two agreed it was a good day for both countries, officials said. Obama also called former President George W. Bush, whose administration was defined by bin Laden’s attacks and the fight against al-Qaida, to inform him of the news.

Afghan President Hamid Karzai said the death of bin Laden proves that the fight against terrorists should be focused not on his nation, but on Pakistan.

Karzai told an assembly of district government officials Monday that it was “a very important day” and that bin Laden had received his due punishment. The hall erupted into applause.

In New York, there were scenes of spontaneous jubilation at Ground Zero, the site where the World Trade Center towers stood until the fateful attacks, bagpipes played Amazing Grace.

“We’ve been waiting a long time for this day,” said Lisa Ramaci, whose husband was a freelance journalist killed in the Iraq war. “I think it’s a relief for New York tonight just in the sense that we had this 10 years of frustration just building and building, wanting this guy dead, and now he is, and you can see how happy people are.”

She was holding a U.S. flag and wearing a T-shirt depicting the twin towers and, in crosshairs, bin Laden. Nearby, a man held up a cardboard sign that read: “Obama 1, Osama 0.”

In Times Square, dozens stood together on the clear spring night and broke into applause when a New York Fire Department SUV drove by, flashed its lights and sounded its siren. A man held an American flag, and others sang The Star-Spangled Banner.

Outside the White House, a crowd began gathering after TV news bulletins that presaged the president’s announcement. The throng grew and within a half-hour had filled the street in front of the White House and begun spilling into Lafayette Park.

“It’s not over, but it’s one battle that’s been won, and it’s a big one,” said Marlene English of Arlington, Va., who lobbies on defense issues. She said she has baked thousands of cookies to send to friends serving in Iraq and Afghanistan over the years and that she was at the White House because they couldn’t be.

As news of the president’s announcement began to filter across the country, fans at a New York Mets-Philadelphia Phillies game in Philadelphia broke into chants of “U-S-A! U-S-A!” in the top of the ninth inning at Citizens Bank Park. Fans all over the stadium were checking their phones and sharing the news.

The chant “U-S-A! U-S-A!” echoed in Dearborn, Mich., a heavily Middle Eastern suburb of Detroit, where a small crowd gathered outside City Hall and waved U.S. flags. Across town, some honked their car horns as they drove along the main street where most of the Arab-American restaurants and shops are located.

Gordon Felt, president of an organization for families of people who were on United Flight 93, which crashed into a Pennsylvania field on Sept. 11, called the announcement of bin Laden’s death “important news for us, and for the world.”

“It cannot ease our pain, or bring back our loved ones,” he said in a statement, but it does bring “a measure of comfort.”

NPR’s Scott Horsley reported from Washington, D.C. from this story, which contains material from The Associated Press. Copyright 2011 National Public Radio.