Editor's note: Gloria Borger is a senior political analyst for CNN, appearing regularly on CNN's "The Situation Room," "AC360°," "John King, USA" and "State of the Union."
(CNN) -- In the aftermath of the killing of Osama bin Laden, a picture is emerging of a long, complex, dangerous and circuitous route to the compound that was the site of the special forces attack.
It was years in the making, involving interviews with high-value detainees like Khalid Sheik Mohammed, who is known as KSM. In the end, though, it all came down to classic espionage work and a judgment call by President Barack Obama, who chose a surgical helicopter attack rather than a bomb. The evidence was good, but it was circumstantial; the CIA believed that the probability of finding bin Laden at the compound was just 60-80 percent. "There was no guarantee by any stretch," says a senior administration official.
The story began many years ago when intelligence officers interviewing high-value detainees in U.S. custody, including KSM and Abu Faraj al Libbi, got some critical leads on couriers "who might be facilitating OBL, (Osama bin Laden)," one U.S. official familiar with the operation said. The detainees confirmed some names. But there was one courier's identity some of the detainees "held out" on; they would only give a nickname.
In fact, when they pushed KSM on the name, he "lied to protect his protege," the official said. The guy, KSM said, "doesn't know anything." And that was the tip-off, said this source: "We knew he was lying, because we already knew he was a KSM protege." As a result, the intelligence operatives pushed harder to try and find out who he was.
As it turns out, he would be the key to tracking down bin Laden.
Without much help, all U.S. intelligence had was the courier's nickname (which they will not share), but no more. "From the nickname, we tried to find out his real name," a senior U.S. official said. After a long search -- and some cold trails -- they finally did, four years ago. But only after what one U.S. official called "classic espionage and intelligence work." In the end, the official said, "we learned his real name from a different part of the world," but he would not elaborate.
The next step was to try to find him. They knew he was in a general area in Pakistan, and they actually once physically viewed him on the street.
But they remained covert. "We couldn't trail him, so we had to set up an elaborate surveillance effort...that finally tracked him back to that compound," the official said. And that was when the intelligence community knew it was on to something potentially huge.
Said this source: "That's when we said, wow , this is different."
White House senior counterterrorism adviser John Brennan described the process this way in a Monday briefing:
"As a result of the information that we had in a very generic way about these couriers and individuals who were cutouts for bin Laden, over time we were able to piece together additional information, get his -- the name he was known by, his nom de guerre, associate that then, eventually, with his real name, associate that then with other things that that real name was associated with, and track it until we got to the compound in Abbottabad."
Once the compound was identified, CIA Director Leon Panetta then "began to hammer folks very hard," one senior administration official said. "I want to know who's living there," he said, and he wanted the information "down to the count...They counted people and told Panetta who was sleeping where."
Indeed, said one White House official, "this was really a CIA-driven operation. The agency put the pieces of the puzzle together."
The information on the house was "not from one source who spilled the beans," said a senior administration official. "The CIA gathered information, saw that something was a little out of whack (at the house), and was tracking the couriers."
So who was in the compound? The CIA knew it was a courier, his wife and kids. Also, there was the courier's brother, his wife and kids.
They eventually were able to figure out there was a third family living in the compound. And the family matched the makeup of bin Laden's -- one of his wives, Alam; son, Khalid; and three other kids. But they didn't know for sure if bin Laden himself was in there.
What really made them suspicious, though, was the whole compound. "The security procedures were very tight, and it was very tightly built," said this source, referring to the trash burning and lack of Internet. "And don't forget: bin Laden is in the construction business."
Yet it wasn't a completely solid case. The administration's problem "was a circumstantial case," said a source familiar with the operation. It was a strong one, though, this official said. "When you added up all of the couriers and the circumstances it was strong." But no slam-dunk, to use some familiar CIA parlance of another era.
This source said that CIA analysts rated the probability of bin Laden living at the house at between 60-80%. One hypothesis was that it could have been his family, without him. U.S. intelligence officials say they have "no evidence that the Pakistanis knew about this (safe house)," according to the senior administration officials. But they have no evidence on that subject one way or the other. "And we would want to know that," this source said.
In the end, he added, "bombing the place was not really a viable option." The president wanted to definitively know if they had hit bin Laden, and that meant a body was needed. Panetta and Obama agreed on that.
Both the president and Panetta considered all the options. As one senior official put it, "you're not absolutely certain he's in there," said this source. "It wasn't like we had 100 percent clear evidence. It was a long stretch between that and certainty."
The deliberations in the White House situation room were about "what's the risk. The benefit of doing something is obvious if this is the guy. So you have to weigh the reward of the risk (of a more targeted approach) versus you drop a bomb on the house, and possibly can't even find DNA evidence."
There was not consensus among all of the senior policymakers whether to go ahead with the mission.
"There were differences of views that were discussed. That's what the president wanted to know -- as well as the different ... courses of action, which are the types of things that you can do that involve an assault on the compound as well as from a standoff position," Brennan said at the briefing. "So this was debated across the board and the president wanted to make sure, at the end, that he had the views of all the principals."
And it was a success. Now, said one of the officials, "the objective is to take this and run, keep our foot on the gas and keep our pressure on the network. We need to use this as a tipping point."
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