It comes in fragments of conversations, snippets of technical data, whispers from foreign agents, and boasts of audacious schemes. Most of it means nothing. Some of it means everything.
In the summer of 2001, the river of information flowing into Washington about possible security threats was cresting.
"There was a lot of chatter in the system," in the words of national security adviser Condoleezza Rice.
But what to make of it all? One of the principal challenges for the government's intelligence analysts is to cull tiny bits of wheat from all the chaff.
Tens of thousands of tips about threats against American targets come in every year. The vast majority, says Defense Secretary Donald H. Rumsfeld, "tend to be eventually discounted as not being valid, or, at the minimum, not being actionable."
U.S. officials are this spring once again evaluating a growing body of intelligence suggesting another large scale Al Qaeda attack may be in the planning for Europe, the Middle East or the United States, government officials said Saturday.
One official, speaking on condition of anonymity, said the "chatter among Al Qaeda types has been increasing in volume" over the last several weeks, suggesting the terrorist network is reconstitutiting itself after a winter of disruptions caused by the U.S. military operations in Afghanistan and widespread arrests across the globe.
Some of the intelligence suggests al Qaida may be agitating for a large-scale terrorist operation but there is no credible evidence as to the method, date or location, and some potential threats have been discounted as not credible, the officials said.
FBI field offices also have been alerted in recent weeks to be aware of possible activity ranging from attacks on Fourth of July events that draw large crowds to assaults on urban sites such as office buildings and apartments. Concern also has been passed on about attacks on supermarkets or shopping centers — but, again, with no specifics and no way determine the likelihood of such attacks.
Often, intelligence experts say, there is no specific warning of the most deadly attacks.
There is great debate in Washington about whether the Bush administration should have known and done more in advance of the Sept. 11 attacks. But all sides agree that the government needs to improve its ability to sort through the mountain of raw intelligence it receives and pull together what is most important — a concept known as fusion.
The intelligence authorization bill passed by Congress in December calls for building up the government's ability to analyze intelligence, increasing the portion of the data that actually gets analyzed and turning it into more useful information.
"You just get a river of this stuff every day," said former CIA Director Robert Gates.
"Most of it is uncorroborated," he added. "Most of it is from a single source, and it's very difficult sometimes to assess both the reliability of the information and its provenance — what kind of authoritativeness there is to it."
L. Paul Bremer, ambassador-at-large for counterterrorism during the Reagan administration, recalls coming in to work every morning to a stack of intelligence reports that might be 8 inches tall.
"And by the end of the day," he adds, "I would've gone through at least that much again."
The types of reports that might come in: Someone in a bar is overheard saying they are going to attack Americans; another country's intelligence agent gets wind of a plot to assassinate the American ambassador; a U.S. businessman sees someone who looks suspicious taking pictures of his company's office in Frankfurt, Germany.
"You've got to figure out what to make of it all," says Bremer. "It's damn hard."
Sometimes, simply a big increase in the sheer volume of intelligence chatter is a clue that something big is afoot and that extra precautions are warranted. That still does not provide the kind of specific information often needed to avert an attack.
In the months before the 1983 terrorist bombing of the Marine barracks in Beirut, Lebanon, the United States had multiple warnings about a possible attack, says Gates, CIA director from 1991 to 1993.
"Everybody knew something was coming, but you didn't know when and you didn't know how," Gates said. "There's a certain parallel to what happened on September 11. Even as disciplined a group as the Marines could only stay on the highest alert for so long." The bombing killed 241 Marine, Navy and Army personnel.
President Bush was advised in the summer of 2001 that Al Qaeda terrorists wanted to hijack planes, and there were FBI reports raising concerns about Arab men taking flight training around the country, but the administration says there was no way to act to prevent the Sept. 11 attacks.
The two key questions about any intelligence report are its credibility and specificity.
In assessing credibility, intelligence analysts must consider whether the source really had access to the type of information being provided, what the source's motives are, whether the source has been truthful in the past, and whether the information can be corroborated.
Even if the information is deemed credible, though, there may not be much the government can do if it is vague.
For example, perhaps a U.S. agent who has infiltrated a terrorist group reports that the chief said "the attack is on," says Bremer.
"That doesn't help you very much if you don't know where, when and against whom."
Sometimes, intelligence clues are recognized for their full significance only once it is too late.
Within hours of the Sept. 11 attacks, the CIA and FBI was focusing new attention on Zacarias Moussaoui, who had been arrested in August while training at a Minnesota flight school and has now been charged as a conspirator in the suicide hijackings.