Obama said there is a risk of terrorist attack on occasion of 10th anniversary of 9/11, and also did not ruled out the likelihood of the dreaded terror outfit al Qaeda launching a revenge attack for the killing of Osama bin Laden.
"The risk of a terrorist attack is always there. And, obviously, on a seminal event like the 10th anniversary of 9/11, that makes us more concerned," the President said in an interview to CNN.
"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," Obama said.
"You know when you have got a one person who is deranged or driven by a hateful ideology, they can do a lot of damage, and it's lot harder to trace those lone wolf operators," he said.
Obama's apparent reference was to recent number of attacks in the US linked to attacks by loners including November 2009, shooting of 13 people in Army's Fort Hood complex in Texas, a botched car bombing in New York's Times Square in 2010 and a December 2009 attempt to bring down a US jetliner, which was also tied to a lone person.
With the US preparing to observe the tenth anniversary of the 9/11 bombings, Obama said federal officials were vigilant to such threats.
Asked if Americans did not have to worry about nuclear or radiological attacks or some other "spectacular events" Obama said, "Look, as the President of the United States, I worry about all of it. But I think the most likely scenario we have to guard against is a lone wolf operation than a large well coordinated terrorist attack. We still have to stay on top of it and we are not letting our guard down."