"Al Qaeda and its affiliates are still committed to striking us in the United States," Mueller told a Senate Appropriations Subcommittee, pointing to plots to bomb New York City subways and the failed Christmas airline attack.
"Home-grown and lone-wolf extremists pose an equally serious threat," the Federal Bureau of Investigation director said, citing the shootings at the sprawling Fort Hood army base in Texas.
Experts have warned that a "lone wolf" -- an extremist acting alone, without connections to an established network like Al-Qaeda -- may be the most difficult threat for authorities to thwart.
"We have also seen US-born extremists plotting to commit terrorism overseas," such as Mumbai attacks planner David Headley, the US-born son of a former Pakistani diplomat and American woman, said Mueller.
"These terrorist threats are diverse, far-reaching and ever-changing, and to combat these threats, the FBI must sustain our overseas contingency operations and engage our intelligence and law enforcement partners both here at home and abroad," he said.