Civil rights advocates protested Thursday in front of the FBI building against a recent memo warning law enforcement officials across the country that would-be terrorists could use almanacs to plan attacks.

In a bulletin sent over the holidays to about 18,000 police organizations, the FBI said terrorists might use almanacs "to assist with target selection and preoperational planning." It urged officers to watch during searches, traffic stops and other investigations for anyone carrying almanacs, especially if the books are annotated in suspicious ways.

According to the Associated Press, the bulletin states "the practice of researching potential targets is consistent with known methods of al Qaeda and other terrorist organizations that seek to maximize the likelihood of operational success through careful planning."

The bulletin has prompted objections from civil liberties advocates who say it might encourage police to arrest or interrogate people based on their reading habits.

"Basically, it makes it potentially illegal for somebody to walk around with their almanac," said Carol Moore of the D.C. Anti-War Network (DAWN) in Washington. Members of the group marched in front of the FBI building Thursday holding up almanacs.

"DAWN is protesting the absurdity of this almanac warning, which infringes on the liberties and privacy of all Americans," Moore added.

Almanacs contain a plethora of data, such as maps and information on the nation's tallest buildings, landmarks, cities and rivers. The books are also loaded with facts about gardening, recipes and planetary systems.

"Founding Father Ben Franklin probably never imagined that the almanac he created could be the subject of an FBI terrorism bulletin," said Nadine Strossen, American Civil Liberties Union president, in a statement.

The almanac alert was part of the FBI's regular intelligence bulletin sent weekly to law enforcement agencies nationwide. FBI spokesman Paul Bresson declined to comment on the bulletin, saying it is an internal memo for law enforcement officials.

"It's a law enforcement-sensitive communication, and I really can't share with you what's in it," he said. "It was never intended to be public."

Bresson said the agency is always trying to give the law enforcement community the best information possible to help them do their jobs.

"What we have done and will continue to do is apprise [law enforcement officials] of all information that helps protect public safety and...we feel it's necessary to arm them with this information," he said.