Counterterrorism officials are tracking threats to the U.S. and Europe from al-Qaida and affiliated groups during the holiday season, authorities said. They have not yet seen evidence of specific plots aimed at the U.S.

The FBI and Homeland Security Department have alerted 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 was sent in a bulletin obtained by The Associated Press. It did not include information about specific plots or intelligence.

On Saturday, a suicide bomber blew himself up on a pedestrian street in Stockholm, Sweden, killing himself and injuring two people. Iraqi officials say that captured insurgents said this week that the suicide bombing was part of attacks being planned by al-Qaida against the U.S. and Europe during the Christmas season.

Even before the revelations from the captured Iraqi insurgents, U.S. counterterrorism officials were tracking threat streams from al-Qaida operatives hiding in Yemen.

There is specific intelligence on other attacks being planned against Europe during the holiday season, according to U.S. officials speaking on condition of anonymity to discuss security matters. While intelligence officials have not uncovered specific details of threats aimed at the U.S., they cannot be ruled out, the officials said.

Still, the 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 U.S. officials on high alert.