"Keeping activities like this secret is the biggest challenge," said the second former U.S. intelligence official.
The vulnerability of being far removed from U.S. protection was seen as another major barrier to the success of the program.
In its initial conception, in the aftermath of the Sept. 11 attacks, the CIA program was seen mainly as an effort to assemble teams capable of carrying out targeted killings. But officials have said that it went through multiple "iterations."
Most recently, the program's focus had shifted toward intelligence collection, officials said, the latest in a series of efforts toward the end of the George W. Bush administration to find Bin Laden.
However, different objectives brought different challenges, officials said, including how to get the right mix of personnel that could operate in the badlands of Pakistan without being captured or exposed.
Former officials declined to say whether the CIA had ever held discussions with Pakistan about setting up hybrid teams with members of the Pakistani military or its main spy service, Inter-Services Intelligence. But one former official said that few officials thought the initiative could succeed solely with U.S. personnel.
"If you're born in Kansas, you're always from Kansas," the former official said. "I don't care you long you grow your beard, you're still from Kansas."
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