As celebrations break out following bin Laden's death, U.S. officials warn of terror group's retaliation.

Bin Laden Killed by CIA-Led SEALs Team, Death Hailed as Blow to Al Qaeda

Years of tracking the world's most-wanted terrorist culminated Sunday afternoon, when a CIA-led Navy SEALs squadron of just a few dozen men stormed Usama bin Laden's compound and killed him. 

President Obama announced the results of the top-secret operation late Sunday night, calling it the most significant blow to Al Qaeda to date. Within hours, the architect of the Sept. 11, 2001, attacks that killed more than 3,000 people was buried at sea. 

Though the president offered only sparse information on the mission and the intelligence that led to it, details have since emerged about the heroic actions of the small, elite team dispatched to Pakistan by an order from the president last week. 

According to officials, a 40-man Navy SEALs squadron raided bin Laden's compound in Abbottabad, Pakistan, at 3:30 p.m. ET on Sunday. As officials described it, the raid was swift -- the team was on the compound for less than 40 minutes and did not run into any local authorities during the firefight. 

At the start of the operation, four U.S.-owned and operated helicopters launched from a base in Afghanistan and dropped about 24 men onto the grounds of the compound. One helicopter suffered a "hard landing" after experiencing a "flight control issue" and had to be destroyed on

At first, bin Laden was asked to surrender. But a military official said he resisted. In the end, he was killed in the ensuing firefight with a bullet to the head. 

No Americans were hurt or killed during the raid. Besides bin Laden, three other men were killed, one of whom is believed to be bin Laden's 24-year-old son. One woman used as a human shield was also killed, and two other women were injured. 

The operation itself stemmed from a tip that came to Obama's desk last August. Specifically, U.S. officials were tracking an Al Qaeda courier in Pakistan, based on information obtained from multiple detainees, and determined the location of the compound in Pakistan where the courier and his brother operated. It was built on a large plot of land, and was heavily secured, with 12-to-18-foot walls topped with barbed wire, officials said. Intelligence analysts determined the compound "was custom-built to hide someone of significance," a senior administration official said. 

After months of analyzing the information, U.S. officials began holding high-level meetings about how to proceed earlier this year. 

On April 29, Obama gave the order to conduct the operation. The actual mission was supposed to happen Saturday night, but it was delayed due to weather. 

The highly trained special forces unit had been practicing the raid a week in advance. 

In announcing the successful completion, Obama said Sunday night that bin Laden's death "is a testament to the greatness of our country." 

"For over two decades, bin Laden has been Al Qaeda's leader and symbol," Obama said. "The death of bin Laden marks the most significant achievement to date in our nation's effort to defeat Al Qaeda." 

In a message to employees, CIA Director Leon Panetta said Monday that "we have rid the world of the most infamous terrorist of our time." 

"Nothing will ever compensate for the pain and suffering inflicted by this mass murderer and his henchmen. But just as evil never rests, neither does good. May the fact that Usama Bin Ladin no longer inhabits the earth be a source of comfort for the thousands of families, here in America and around the globe, who mourn the victims of Al Qaeda's barbarity," he wrote. 

He added that while bin Laden is dead, "Al Qaeda is not." 

In the wake of bin Laden's death, authorities around the world are being urged to take security precautions. One source said officials are concerned bin Laden's death could incite violence or terrorist acts against U.S. personnel overseas. 

The State Department issued a travel alert for U.S. citizens abroad overnight, citing "the enhanced potential for anti-American violence given recent counterterrorism activity in Pakistan." 

Obama said Americans must continue to be "vigilant." But he said the death of the architect of the deadliest terror attack on U.S. soil should be welcomed around the world. 

"Bin Laden was not a Muslim leader. He was a mass murderer of Muslims," Obama said. "So his demise should be welcomed by all who believe in peace and human dignity." 

Fox News' Jennifer Griffin, Justin Fishel and Catherine Herridge contributed to this report.

Osama bin Laden killed by Navy SEALs: Details emerge

US national security sources describe how Osama bin Laden was tracked down.

A pedestrian walks past a TV news report about the killing of Osama Bin Laden, at a railway station in Seoul on May 2, 2011. (Jung Yeon-Je/AFP/Getty Images)

BOSTON — The mission: Get Osama bin Laden.

Acting on intelligence painstakingly gathered since August, U.S. attack helicopters crossed from Afghanistan into Pakistan on Sunday and inserted a Navy SEALs unit known as “Team Six.” Within an hour, the legendary, elite unit had surrounded a high-walled compound and shot the mastermind of the Sept. 11 attacks.

Inside the labyrinth of the compound, which was located about 20 miles north of Islamabad and not far from a Pakistani military training facility, the SEALs unit came face to face with bin Laden and in an intense fire fight, the Al Qaeda leader was shot in the head and killed.

The unit credited with the assault on the compound is a “Tier One” special operations counterterrorism unit that is highly secretive and exists outside the traditional military hierarchy. They are officially named the U.S. Naval Special Warfare Development Group, but more informally referred to by their former name, which is “Team Six.”

Based on interviews with two national security sources, a picture is emerging of a daring and stunningly successful military operation that finally brought bin Laden to justice nearly 10 years after the attacks on the World Trade Center and the Pentagon.

Those sources say that the critical break in the case came when Al Qaeda fighters detained at Guatanamo produced information about a courier who carried messages to Bin Laden. That piece of information led to locating the compound in which bin Laden was staying.

“The death of bin Laden marks the most significant victory to date in our effort to defeat Al Qaeda,” said U.S. President Barack Obama last night, speaking from the White House.

But Obama and national security officials were quick to warn that the struggle against terrorism is far from over and that Al Qaeda will continue to try to carry out its self-declared war against America.

“His death does not mark the end of our effort … We must and we will remain vigilant at home and abroad,” Obama said.

One national security source said that in fact the killing of bin Laden could open up intelligence avenues that Obama’s national security team hopes will allow them to target other senior leaders of Al Qaeda. The focus would be the Egyptian Ayman al Zawahiri who was bin Laden’s deputy and is now presumed to be the new leader of Al Qaeda, a movement believe to have inspired militant cells in Yemen, Somalia, the Philippines, Indonesia and dozens of other predominantly Muslim countries around the world. Meanwhile, the source added that there is grave concern that Al Qaeda may try to carry out an attack soon to prove it is still active.

The mission was a fulfillment of Obama’s vow — which was controversial when he first made it as a candidate for his party’s nomination — that he would take the fight to bring bin Laden to justice into Pakistan, if necessary.

Last night, Obama addressed this, saying, “I have made it repeatedly clear that we would take action within Pakistan if we knew where he was. That is what we have done.”

Obama added that the operation was done in concert with Pakistan and that he had called Pakistani President Asif Ali Zardari. He said his national security team had called their counterparts in Pakistan.

“They agreed this is a good and historic day for both of our nations. And going forward it is essential that Pakistan join us in the fight against Al Qaeda and against terrorism,” Obama said.

But, Obama was also careful to add, “The U.S. is not and never will be at war with Islam … I made clear, just as President Bush did after 9/11, that our war is not with Islam. His demise should be welcomed by all who believe in peace and human dignity.”

The intelligence operation that tracked down bin Laden outside of the Pakistani capital was more than nine months in the making. It led to the order Sunday for an assault on a sprawling compound outside of Islamabad, where he was gunned down by Navy SEALs as part of a Joint Special Operations Command Force.

Bin Laden's body is in U.S. custody and FBI forensic teams have confirmed that the DNA proves that it is the body of the Al Qaeda leader, national security sources confirmed.

The raid in Abbottabad, just north of the Pakistani capital Islamabad, came out of a vigorous commitment by Obama to push the CIA to make bringing bin Laden to justice the No. 1 priority.

It also came out of a dramatic increase in intelligence assets in Afghanistan and a more aggressive intelligence effort to use drone spy planes. There was also a coordinated effort with Pakistani forces to track down Al Qaeda’s leadership.

One national security source said that it is believed those intelligence assets pushed bin Laden out of the mountains along the border of Pakistan and Afghanistan and deeper into Pakistan.

“There was just relentless pressure in the last two years. We really stepped it up and we believe that was what led to this successful mission and this great day where all of us can finally say we brought bin Laden to justice,” said the well-placed national security official.