It is perhaps one of President George W. Bush's more frustrating failures, and one of the most difficult tasks he'll hand over to his successor.
More than seven years after the attacks of Sept. 11, 2001, Osama bin Laden, the man Bush once said he wanted captured "dead or alive," has not been found.
President-elect Barack Obama has said that he plans to renew the U.S. commitment to tracking down the al Qaeda leader.
"I think it is a top priority for us to stamp out al Qaeda once and for all, Obama told CBS's "60 Minutes" Sunday. "And I think capturing or killing bin Laden is a critical aspect of stamping out al Qaeda. He is not just a symbol, he's also the operational leader of an organization that is planning attacks against U.S. targets."
As soon as Obama takes office in January, he has said he will begin work on a plan to draw down U.S. troops in Iraq and beef up U.S. troop numbers in Afghanistan.
"Particularly in light of the problems that we're having in Afghanistan, which has continued to worsen. We've got to shore up those efforts," Obama told "60 Minutes."
During the presidential campaign, Obama promised he would refocus the military's fight to taking on the resurgent Taliban in Afghanistan.
His promise to send two additional combat brigades, or roughly 7,000 troops, to Afghanistan seems small now when compared to the request by top U.S. military commanders there for as many as 20,000 U.S. forces.
But finding bin Laden won't be easy, and several former CIA operatives warn that sending more troops could have severe consequences, won't put a dent in finding the al Qaeda leader and could lead the nation to another war without end.
Osama Has 'Completely Disappeared'
"He's completely disappeared," former CIA operative Robert Baer told ABCNews.com about bin Laden.
"I asked my CIA colleagues who have been on the hunt for him, and what surprised me was, no one was quite sure. Half assumed he was alive, and half assumed he was dead," Baer said. "Obviously, they have lost track of this guy completely."
Baer, who was the inspiration for the George Clooney character in the film "Syriana," said intelligence gathering in the mountainous region is "virtually impossible."
The warring Pashtun tribes along the border of Afghanistan and Pakistan complicate military operations in the region, and more troops won't help the U.S. capture bin Laden.
"It would take a million U.S. soldiers to go up there to subdue that area," Baer said.
Where Is Osama bin Laden?
Beyond the military fight along the border between Afghanistan and Pakistan, the CIA has for years engaged in an aerial war against suspected terrorist targets inside Pakistan using missile-armed Predator drones.
Since September, the CIA has substantially increased the number of those missile attacks, and some have successfully targeted senior al Qaeda operatives. But bin Laden remains an elusive target.
"If we did find bin Laden, it would be more out of luck than anything else," said Baer.
In late 2001, bin Laden was nearly captured in a battle with U.S. forces near Tora Bora, Afghanistan. However, the U.S. has since lost track of him, believing he is now hiding somewhere in the remote region around the Afghanistan and Pakistan border.
"Anyone familiar with the Afghan-Pakistan border area knows how rugged and inaccessible it is," CIA Director Gen. Michael Hayden said Thursday.
"Beyond that remoteness, the sheer challenge of surveying every square mile of that inhospitable and dangerous region, part of the explanation for his survival lies in the fact that he has worked to avoid detection," Hayden said.
Former senior CIA official Michael Scheuer, who led a special CIA unit whose mission was to hunt bin Laden, argues it's unlikely the al Qaeda leader will be caught.
"We tend to forget that he's been at this for 25 years," Scheuer told ABCNews.com. "He's quite experienced as an insurgent in terms of not being found."
$25 Million Reward Unclaimed
The U.S. government has offered a $25 million reward for information leading to bin Laden. But Scheuer points out, despite the fact Afghanistan is one of the world's poorest nations, no one has claimed the money.
Scheuer is convinced bin Laden is alive.
"He lives among the Pashtun tribes, and they have a tribal code of conduct that, once you accept someone as your guest, you then have to protect them with your life," he said. "And so there's very few people who are going to turn over Osama bin Laden."
Obama: 'We Will Kill Bin Laden'
If bin Laden is captured or killed, what isn't known is how his followers around the world will react.
"I think it would be a key transformative political event because of the symbolic importance of it," said Samuel Brannen, an international security expert at the Center for Strategic and International Studies in Washington, D.C.
"But we don't even know what it would really do. Would he be a martyr or would this be a leaderless movement?"
During his campaign for the White House, Obama pledged to end the war in Iraq and renew the nation's focus on bin Laden and Afghanistan, arguing the Bush administration's focus on Iraq diverted resources away from the war on terror.
"We will kill bin Laden. We will crush al Qaeda," Obama promised during an Oct. 7 debate. "That has to be our biggest national security priority."
But putting more troops in Afghanistan won't relieve the overall strain on U.S. military forces and their families.
"It looks like Obama's focus is on putting the bulk of the troops in Afghanistan, with a smaller operation in Iraq, which is really just his and President Bush's policies flipped," Brannen said.
Currently there are 31,000 U.S. troops already in Afghanistan. NATO forces represent about another 30,000 troops.
'Afghanistan Is the Graveyard of Empires'
The challenges facing the U.S. in Afghanistan are daunting: keep the increasingly unpopular Karzai government in power, rebuild the Afghan economy and build a communications and transportation infrastructure.
More important, the U.S. and allies have the task of trying to find and destroy al Qaeda, eradicating the world's largest heroin industry and trying to defeat the Taliban, which has established a base in Pakistan, the Muslim world's only nuclear-armed arsenal.
"Al Qaeda's base in Pakistan is the single most important factor today in the group's resilience and its ability to threaten the West," Hayden said.
Baer argues sending more troops into Afghanistan "is a huge mistake" that could escalate the problem.
"It's going to push the chaos into Pakistan," Baer said. "The cliché is, Afghanistan is the graveyard of empires."