The team of Navy SEALs that carried out the mission to kill Osama bin Laden Monday trained for months before loading into helicopters and descending on the world's most wanted terrorist.

They worked in calculated coordination, taking just 40 minutes to take hold of bin Laden's home in Abbottabad, an upscale neighborhood in Pakistan. Bin Laden died of a bullet wound to the head.

"The training they do is very rigorous," former SEAL Mike Smith told WPTZ News. "Their mentality is to finish the mission."

Shortly after officials made the decision in March to attack bin Laden via helicopter assault, the Naval Special Warfare Development Group (also known as Team Six) started holding dry runs at training facilities on both American coasts, the New York Times reported. The team of about 40 SEALs, described by former SEALs as "the best of the best," moved to Afghanistan in April, according to the Times of India, where they continued their work in an isolated part of the Bagram Air Base.

Facilities in both countries were made up to resemble bin Laden's compound, a three-story house ringed by 12-foot-high concrete walls, topped with barbed wire and protected by two security fences. They were not told who their target would be.

"The word is that when they heard that bin Laden was their target, there was a huge cheer that went up," Eric Greitens, a former SEAL and author of "The Heart and the Fist," said on NBC's TODAY. "These guys were excited for the mission, they had been practicing for months."

SEAL Team Six, known at their home base in Virginia as "DevGru," is part of the Joint Special Operations Command (JSOC), which specializes in covert operations. JSOC has been active in Pakistan for several years, but, because of the clandestine nature of many missions, doesn't garner much attention unless an operation "goes horribly wrong," the Times of India reported. Scores of men have died, but their deaths have been announced in a way to create the impression they were killed in Afghanistan.

The SEALs won't confirm Team Six carried out the attack, but their current chief, Rear Adm. Edward Winters of the Naval Special Warfare Command in California, sent an email congratualting his forces and cautioning them to keep their mouths shut.

"Today we should all be proud. That handful of courageous men, of strong will and character, have changed the course of history," he wrote, according to MSNBC, "Be extremely careful about operational security ... The fight is not over."

The SEALs chosen for such a special mission would likely be "tapped by superiors because of a skill that sets them apart," former SEALs told CNN. They must also be able to take over for teammates should anyone be hurt or killed.

"They need to go far beyond just being a skilled warrior," said Brandon Tyler Webb, a former SEAL who ran the sniper program at the Navy Special Warfare Command and was part of combat missions in Iraq and Afghanistan. "Getting on a special team means you've established yourself as a mature and steady operator with a real world track record of high-stakes, sensitive missions."

Chris Heben, a former SEAL with 10 years of experience carrying out missions in Africa, the Middle East and Afghanistan, called SEAL training, which lasts between 18 and 24 months, "the ultimate test for a guy." Training is psychological and social as well as physical.

About 90 percent of those who set out to become Navy SEALs drop out before completing training, he said.

"Every day is like climbing Mount Everest," Heben said. "You just keep doing what's in front of you. You don't look up."

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