The FBI and Homeland Security Department are alerting state and local law enforcement to be wary of suspicious behavior and to regularly change security measures to interfere with any terrorist plans. The warning, sent in a bulletin obtained by The Associated Press, does not include information about specific plots.

A spate of attempted attacks against the U.S. in the past year — particularly the Nigerian man charged with trying to take down an airplane last Christmas — has officials on high alert.

Captured insurgents in Iraq claim al-Qaida is planning attacks in the U.S. and Europe during the holiday season.