WASHINGTON -- They come in as a steady barrage, about 30,000 each year, intelligence tips suggesting a new potential threat to the United States. Yesterday, another surfaced publicly: a threat to shopping malls in Chicago and Los Angeles during the holiday season.
But the rush to discount the threat, first reported by ABC News, highlighted what is perhaps the most difficult challenge counterterrorism analysts contend with: determining which threats are real.
"Its very tough," said Andrew Apollony, a senior FBI counterterrorism official. "It's not an exact science."
With 30,000 threat tips each year, he added, "it's crucial ... to be able to discriminate between the wheat and the chaff."
Baltimore had its own brush with the danger of noncredible threats two years ago, when word of a possible terrorist plot to blow up one of Baltimore's tunnels shut down Interstate 95. The informant turned out to be wrong.
In the case of yesterday's threat, FBI officials said in a statement, they distributed the information to police agencies "out of an abundance of caution," but "there is no information to state this is a credible threat."
The FBI report said that al-Qaida planned to attack malls in Chicago and Los Angeles "during the 2007 Christmas season" to disrupt the U.S. economy.
Shopping malls have long been considered potential terrorist targets, particularly during the holiday season, noted retired Army Col. David McIntyre, who directs a homeland security think tank at Texas A&M; University. A pre-Sept. 11 war-gaming exercise, known as Dark Winter, imagined a smallpox attack in shopping malls in three states at Christmastime.
"There is a certain logic to that, even if you had no details," he said. "That's where large numbers of people congregate."
Holiday terrorist threats are also not new. In 2001, "shoe bomber" Richard Reid was apprehended just before Christmas. In 2003, a terrorist threat to U.S.-bound flights from overseas raised the national threat level and grounded six flights to Los Angeles on Christmas Eve.
Intelligence and counterterrorism officials are ultra-sensitive to holiday threats, said John Rollins, a former intelligence official at the FBI and Homeland Security.
"It's a tense time," Rollins said. "You recognize it would be an ideal time to hit the country."
Terrorism threats are particularly difficult to pin down, however, said John Gannon, the CIA's former deputy director for intelligence, because "almost all the information related to terrorist incidents is fragmentary." Each piece of information, which might come from different sources, must be verified separately.
Key to that process, he said, is examining the source of the information: Does the source have a strong track record for providing accurate information?
It's also important to understand whether the source has firsthand information. In the case of the shopping mall threat, the report emphasized that the information was secondhand.
Another step analysts take is to try to cross-reference the information with other sources, officials said. They look at whether other terrorist groups have spoken about similar activities.
The National Counterterrorism Center now conducts video conferences with the intelligence agencies and the White House on emerging threat information three times a day.
Analysts also look for indicators suggesting someone might be plotting, such as concealing their movements, their travel or their financial transactions, said the FBI's Apollony.
Officials said they also scrutinize the alleged plot for holes, such as a timeline that does not appear realistic, or a lack of training to carry out the plot.
With the Baltimore tunnel threat, the tipster was specific enough to be taken seriously, providing names of people he said were plotting to blow up one of the Baltimore tunnels.
But the informant, an Egyptian who had lived in Baltimore, performed poorly on a polygraph test, and his story could not be corroborated, according to a report.
Later, others in the immigrant community said the informant, who had been detained on immigration violations, had sought revenge against those who failed to help him re-enter the United States.
Sun reporter Gus Sentementes contributed to this article.