With the Christmas season in full swing, wary U.S. Homeland Security Department officials have been particularly alert to terror attacks from outside the country.
However, U.S. Attorney General Eric Holder said much of the terror threat within the U.S. is coming from its citizens.
The increased threat may find airline passengers more accepting of invasive searches at U.S. airports than they were just a month ago.
Since the controversial pat-downs and body scanners went into effect in November, there have been fewer complaints and little fuss. This month marks the anniversary of the Christmas Day bomb plot attempt by would-be underwear bomber Umar Farouk Abdulmutallab, one reason why travelers may be complaining less.
Also, news reports of homegrown terrorists have been on the rise. Fifty American citizens have been arrested for plotting or perpetrating terror just in the last 18 months.
"The threat has changed from simply worrying about foreigners coming here to worrying about people in the United States," Holder said. "American citizens raised here, born here. You didn't worry about this even two years ago."
Holder worries about how many of these radicalized Americans are linked to the American-born terrorist cleric Anwar Al-Awlaki.
"He's an extremely dangerous man," Holder said of the Yemen-based Awlaki. "He has shown a desire to harm the United States. A desire to strike the homeland of the United States. He's certainly on the list of the people who worry me the most."
"The threat is real," he added. "The threat is different. The threat is constant."
What remains constant are the terror threats in Britain. Security officials have reportedly been keeping constant track of some 200 terror cells.
After one cell appeared close to acting on its threat, British security officials moved in and arrested 12 men on Monday. The men were apparently targeting landmarks and public spaces in three or four cities. They may have even been targeting the British Parliament building.
The suspects were all Muslim men of Bangladeshi or Pakistani origin, varying in age from 17 to 28.
Tuesday, in Rome, a bomb was found on a subway but with no detonator. Law enforcement officials determined it couldn't have exploded.
Meanwhile, in Washington, D.C., police have stepped up security on its Metro transit system after two separate Muslim men threatened to detonate bombs aboard it. On Tuesday, screeners began random checks of riders bags for explosives.