US President Barack Obama warned that "lone wolf" terrorists are the biggest security threat to the United States ahead of the anniversary of the 2001 al-Qaeda attacks, DPA reported.
"The biggest concern we have right now is not the launching of a major terrorist operation," he said in an interview with broadcaster CNN late Tuesday.
"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."
"There may be a little extra vigilance during 9-11," he said, in reference to the 10th anniversary of the September 11 in New York and Washington that left around 3,500 dead.
On July 22, a Norwegian right-wing extremist allegedly set off bombs in Oslo before going on a shooting spree, killing 77 people.
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