WASHINGTON (AP) -- U.S. intelligence in the struggle against terrorism comes in many forms, maddeningly general, improbably precise, a game of sorts with vast consequences for winner and loser.
It's a satellite image showing tribesmen gathering in a remote area where none should be - the photograph so clear you can see the caliber of ammunition they are carrying.
It's a snatched bit of conversation between two terrorist leaders, overheard by a trusted source the terrorists don't realize is listening.
It's a stolen diplomatic cable. That's right, we steal.
Each of these sources and a multitude of others can become the tips that put an entire nation on alert, as a single tip has done from a single source just before the 10th anniversary of the Sept. 11 attacks.
Here are some questions and answers about the gathering and use of intelligence:
Q: What is intel?
A: Simply put, it is information from anywhere that the U.S. can use to its advantage in the world, either in pursuit of its goals or to protect its people. It can be as basic as a diplomat reading a local newspaper and passing on something interesting to a superior in an embassy or Washington. But it gets much more sophisticated and aggressive than that.
In counterterrorism, bits and pieces of information form a messy picture like an impressionist painting. Those collecting the signs and signals look for a pattern, eventually an image, that gives them a target to go after or tells them which target to protect.
Q: What are the main forms of intel?
A: Perhaps the spookiest is measurement intelligence, known as "masint," using far-away technology to get extremely up close and personal. The U.S. is capable of placing a sensor near a suspected terrorist hideout that can count the number of heartbeats in a room and gauge how fast they're beating. There are even efforts to understand what a "guilty" heartbeat pattern might be.
Masint, working in combination with other kinds of intelligence-gathering, was one of the clinchers in the raid that killed Osama bin Laden. The U.S. measured the approximate height of the tall man who paced beneath the trellis outside the kitchen in a Pakistani compound, to help determine bin Laden was that man.
Then there is human intelligence, or "humint," which has been around since the dawn of spycraft and is still vital. That's the tipster you cultivate and pay, or perhaps the unproven one who simply walks into a U.S. embassy and offers information.
"Sigint," or signals intelligence, monitors or otherwise exploits radio, telephone or satellite phone transmissions. "Imint" is imagery intelligence that includes photographs showing the outlines of a nuclear power plant, obscured in desert sands, or a terrorist training base tucked in a mountainside.
Cybertracking is a newer tool, pursuing terrorists who use computers either to attack a computer network or, more often, to organize how their own human network would launch a physical attack.
Q: What happens to this mountain of raw material?
A: Each of those streams of data is captured by a multibillion-dollar worldwide network of U.S. satellites, armed drones, static blimps and high-flying spy aircraft, manned or unmanned, that suck up so much sound it takes massive computers to crunch all the data and help analysts overwhelmed by the technical information to find meaningful clues.
Sometimes these streams are collected by U.S. operatives - Americans or those in U.S. employ - who go behind enemy lines or into enemy homes, leaving behind the sensors that will provide the data.
True to its name, the Central Intelligence Agency is an "all-source" organization using all means.
Q: How do all these eyes and ears work together?
A: Sometimes they don't. After a Nigerian allegedly tried to bring down a Detroit-bound airliner on Christmas Day almost two years ago, it emerged that his father had warned U.S. diplomats about his son's possible terrorist sympathies, but that tip was lost in the blizzard of counterintelligence.
But in the bin Laden raid, a human source led to the compound in the Pakistani army town of Abbottabad. Signals intelligence monitored for phone calls emanating from there, and found none, because bin Laden forbade them, hoping to evade detection by just such technical means. Masint was derived from the imagery taken by drones and satellites.
All of this helped to convince CIA analysts they had found their man and persuade President Barack Obama to approve a dangerous and diplomatically risky raid into Pakistani sovereign territory.
Q: How is all of this intel sorted through so that the real threats are detected and averted?
A: The ever-present risk is that they won't be. One of the failings of pre-9/11 counterintelligence was that information was jealously guarded by the individual intelligence agencies. Word of a potential plot to fly planes into U.S. landmarks was received by one agency. Another agency had word terrorists might be attending flight school. Each organization kept to itself the dots of information that, when connected, could have revealed the larger pattern of a massive terrorist plot.
Before raw data and human tips can be called "intelligence," they must be analyzed, and if possible, corroborated. The CIA alone has 2,500 people in its Directorate of Intelligence devoted to that task. There are thousands more across the 16 intelligence agencies, sifting raw data, and cross-comparing within their own agencies, and with others, to spot a pattern.
Q: What does it mean to receive - and warn the public about - a credible and specific but unconfirmed threat, as in the latest case?
A: A credible threat means it was heard from a trusted source, not just anyone. Specific means the U.S. has details about when, where or how an attack might unfold. When a threat is specific and credible but unconfirmed, that means intelligence officials haven't been able to validate the information even though they trust the source who gave it to them.
This particular threat is not unusual, but it's being taken so seriously because it comes days before the 10th anniversary of 9/11, a date al-Qaida has eyed for attack.
Q: How is the intel being analyzed in the latest threat?
A: Right now, teams of analysts are combing through information gleaned from one trusted source, who heard that a small group of attackers, perhaps from Pakistan, might blow up a car bomb in New York or Washington. One or all of the attackers might be from Pakistan. Newly minted al-Qaida leader Ayman al-Zawahri might be behind it.
These analysts are looking for anything to corroborate that report in the reams of information they've gathered tracking travelers to the U.S. from Pakistan. U.S. spies overseas will be going back to their sources to see if anyone has heard the same rumor.
Everything unfolds in a hurry.
"You don't have the luxury of vetting the source and then disseminating the information," said Phillip Mudd, a former top counterterrorist official at the CIA and the FBI. "You have to tell everyone what you heard and then try to prove the information is legitimate.
He said he would be directing analysts to pore over everything that can be gleaned from flight and passport logs of potential foreign suspects who have traveled to the U.S.
"Figuring out who would-be attackers are, or even whether they exist, could take months, where the drumbeat of national security wants answers in minutes or days," Mudd said.
Q: What rules do U.S. intelligence-gatherers have to play by?
A: Looser rules than for most people.
The CIA operates under the U.S. law known as Title 50 - literally a license to break laws in foreign countries, by committing espionage, persuading a local official to commit treason, or in extreme circumstances, to go into a foreign country and target a suspect for killing or capture. Title 50 operations are covert, meaning the U.S. never intends to acknowledge them. Other intelligence agencies, such as the eavesdropping National Security Agency and the new Cyber Command, routinely operate under Title 50 as well.
Q: Ultimately, doesn't it all come down to getting lucky - or unlucky?
A: It can feel that way. There is a favorite expression among intelligence officials, memorably if confusingly uttered by former Defense Secretary Donald H. Rumsfeld, that captures the essence of their work:
"As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know."
Associated Press writer Eileen Sullivan contributed to this report.