WASHINGTON -- "Lone wolf" terror plots are on the rise and are "much more difficult to intercept" than the major coordinated effort that went into the Sept. 11 terrorist attacks, Homeland Security Secretary Janet Napolitano said Wednesday.
In a speech at a Chamber of Commerce event, Napolitano said the U.S. has a "layered system of security that would give us multiple ways to deter" an attack like the one a decade ago in which airplanes were weaponized.
"What we see now is smaller plots," she said. "We are also seeing a rise of activities by individuals who are actually in the country, and they are acting by themselves and that kind of attack is the most difficult to prevent because there is nothing to intercept."
Napolitano's comments echoed what President Obama said in an interview Tuesday in discussing the 10-year anniversary of Sept. 11.
The FBI and Homeland Security sent a bulletin late Tuesday to local and state law enforcement agencies and obtained by Fox News that warns of potential lone wolf threats to the U.S., specifically "small arms operations."
The six-page document cites specific examples, including a July incident where a U.S. soldier is accused of plotting to attack Fort Hood with improvised explosive devices (IEDs) and guns. And the attack in Norway, where Anders Brevik, described as a right-wing extremist, car-bombed government buildings and opened fire on a camp, killing 69.
A Homeland Security official told Fox News that the bulletin is not driven by new intelligence, but the growing concern about lone wolf operations.
With the nation preparing to observe the 10th anniversary of hijacked airliners crashing in New York and Washington and in the Pennsylvania countryside, Obama said the government is in a state of heightened awareness.
"The biggest concern we have right now is not the launching of a major terrorist operation, although that risk is always there," the president said.
"The risk that we're especially concerned over right now is the lone wolf terrorist, somebody with a single weapon being able to carry out wide-scale massacres of the sort that we saw in Norway recently," he said. "You know, when you've got one person who is deranged or driven by a hateful ideology, they can do a lot of damage, and it's a lot harder to trace those lone wolf operators."
In Norway, Anders Behring Breivik, 32, was charged with killing 69 people last month at a youth camp. A lawyer for Breivik says his client believes the massacre was necessary to save Norway and Europe from Muslims and punish politicians who have embraced multiculturalism.
In recent years the U.S. has endured terror-linked attacks that authorities believe were carried out by a single person. In November 2009, 13 people were killed at Fort Hood, Texas, in a shooting that led to charges against an Army psychiatrist who authorities allege had become an Islamic extremist.
A botched car bomb in New York's Times Square in 2010 and a Dec. 25, 2009, attempt to bring down a U.S.-bound jetliner with a bomb also were tied by authorities to one person in each instance.
In the CNN interview, Obama said the government continues to monitor and gather information about potential terror plots, even though Al Qaeda's capabilities have been degraded. Al Qaeda leader Usama bin Laden was killed by Navy SEALs during a raid in Pakistan last May.
Asked if Americans do not have to worry about a nuclear or radiological attack or some other "spectacular" event, Obama said: "Look, as president of the United States, I worry about all of it.
But I think the most likely scenario that we have to guard against right now ends up being more of a lone wolf operation than a large, well-coordinated terrorist attack. We still have to stay on top of it, though, and we're never letting our guard down. That's part of our job."
Fox News' Trish Turner and The Associated Press contributed to this report.