In unusually candid remarks, Michael Leiter, director of the National Counterterrorism Centre, said that the nation’s defences would be probably be breached by a home-grown radical, after a year-long period containing several failed or thwarted attacks that had seen the most intense terror activity since September 11, 2001.
“Although we aim for perfection, perfection will not be achieved. Just like any other endeavour, we will not stop all the attacks,” he said.
“If there is an attack, it may well be tragic. Innocent lives will be lost. But we still have to be honest, and we have to be honest that some things will get through.”
He said: “To say that we will not successfully defend against all attacks is certainly not to say that we are not trying to stop all attacks. We are.”
The FBI last week arrested Mohamed Osman Mohamud, a 19-year-old Somali-born American, for plotting to detonate a bomb as thousands of people attended the lighting of the Christmas tree in the centre of Portland, Oregon.
Speaking at the Centre for Strategic and International Studies in Washington, Mr Leiter, who advises the US government on the terrorist threat, said: “In this era of a more complicated threat, a more diverse threat and lower-scale attacks to include individuals who have been radicalised here in the homeland, stopping all the attacks has become that much harder.”
The threat has risen in part because of the increased involvement of the Yemen-based al-Qaeda in the Arabian Peninsula, and Anwar al-Awlaki, an American-born radical cleric.
Known as AQAP, it has pursued smaller attacks perpetrated by lone operators which have complicated the challenges facing the US security services still battling the threat of another attack on the scale of September 11.
AQAP and Al-Awlaki, now living in Yemen, have been linked to Maj Nidal Hasan, an army psychiatrist, is accused of killing 13 people during a shooting spree at Fort Hood, Texas in November, 2009, and to Umar Farouk Abdul Mutallab, the Nigerian student suspected of the failed attempt to blow up a flight headed for Detroit last Christmas.