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Just days before the one-year anniversary of Usama bin Laden's death, federal authorities are telling partners around the country there is no specific, credible threat to the U.S. homeland but they remain concerned "lone wolf" terrorists could use the date to avenge the former Al Qaeda leader's death.

In an intelligence bulletin issued late Wednesday, the FBI, Department of Homeland Security and U.S. Northern Command note that terrorist groups such as Al-Shabaab in Somalia, Northern Africa's Al Qaeda in the Islamic Maghreb, and the Pakistani Taliban have called for revenge against the United States for killing bin Laden during the May 1, 2011, raid on his hideout in Pakistan.

The bulletin says Al Qaeda or its affiliates would view an attack "on this anniversary as a symbolic victory," especially in the wake of losses suffered by Al Qaeda through U.S. drone attacks and other efforts overseas.

In addition, according to the bulletin, authorities remain concerned that so-called "lone-wolf" extremists not already identified "will execute attacks with little or no warning on or about the anniversary of bin Laden's death."

Meanwhile, the U.S. Embassy in Pakistan issued a warning to Americans in Islamabad, telling them to avoid restaurants and markets for the next two weeks. In a message posted online, the embassy cited "security concerns," but it's unclear if the warning is related to the one-year anniversary of bin Laden's death.

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A report issued Wednesday by the European Union, looking at how terrorism has changed in Europe over the past year, says the "threat has evolved" since the deaths of bin Laden and other terrorist leaders.

"[L]one actors or small EU-based groups are becoming increasingly prominent, as is the Internet as a key facilitator for terrorism-related activities," Interpol's E.U. Terrorist Situation and Trend Report says. "Al Qaeda's call for individual violent jihad through the execution of small-scale attacks may result in an increase in such attacks. The more Al Qaeda's core is under pressure, and the more difficult it becomes to prepare large scale attacks, the more Al Qaeda will try to recruit individual supporters in the West to plan and execute attacks."

The EU report cites a "solo terrorist" of Moroccan descent, who adhered to Al Qaeda ideology and was arrested in August 2011 for "planning to poison the water supplies of tourist locations in Spain, in retaliation for the death of bin Laden."

As for the U.S. government bulletin issued Wednesday, it cites Al Qaeda's fixation since at least 2010 with launching attacks on symbolic dates. In addition, the bulletin suggests recent controversies over the desecration of bodies in Afghanistan and the burning of Korans could further inflame passions among extremists.

Nevertheless, the bulletin says authorities "continue to assess that operational readiness remains the driving factor behind the timing of Al Qaeda attacks," and authorities do not expect jihadist messages online calling for revenge "to accelerate or motivate attack plotting."

"[W]e have not detected signs of homeland plots by (known) groups in the intervening months" since bin Laden's death, the bulletin reads.

Over the past year, bulletins issued by the U.S. intelligence community near anniversaries or holidays have repeatedly referenced calls to avenge bin Laden's death.

Asked about Wednesday's bulletin, a U.S. official would only reiterate that authorities "have no indication of a credible threat or plots against the U.S. that coincide with the one year anniversary of bin Laden's death."

During a Senate hearing Wednesday morning, Homeland Security Secretary Janet Napolitano said DHS efforts "to get information, tools and resources out of Washington, D.C., and into the hands of state and local officials and first responders" have "led to significant advances" in protecting the homeland.

Still, the bulletin issued Wednesday says authorities remain concerned about the possibility that the group behind the Christmas Day 2009 bombing attempt and the 2010 cargo-bomb plot, Al Qaeda in the Arabian Peninsula, "intends to advance plots along multiple fronts, including renewed plots to target Western aviation."

The bulletin notes the group's continued desire to recruit Westerners to its ranks.