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 Wednesday, 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 Pakistan and 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.

"We are concerned these terrorists may seek to exploit the likely significant psychological impact of an attack targeting mass gatherings in large metropolitan areas during the 2010 holiday season, which has symbolic importance to many in the United States," the joint FBI and Homeland Security bulletin said.

The Homeland Security Department would not comment on specific intelligence. But the U.S. government has been warning Americans since October to be vigilant when they travel to and within Europe because of terror threats directed at the region.

Earlier this month a Portland man was caught in an FBI sting operation as he allegedly planned to set off a bomb at a crowded Christmas tree lighting ceremony. And in October, al-Qaida's Yemeni offshoot — which also claimed responsibility for the Christmas airliner attack — tried to blow up two cargo planes over the U.S. That plot was foiled after officials received a tip from Saudi intelligence.

Counterterrorism officials said they could not discount potential threats from other terror groups, such as al-Qaida's offshoots in Iraq and the Islamic Maghreb, Pakistan and Somalia.

"Al-Qaida and other terrorist groups continue to seek innovative ways to conduct attacks and circumvent security procedures, and we remain concerned that the holiday season provides attractive opportunities for terrorists to target the homeland," the intelligence bulletin said.


Associated Press writer Qassim Abdul-Zahra in Baghdad contributed to this report.