Ridge says new measures have prevented more than 100 attacks
Senior members of the Bush administration have said there is a very real potential that there will be another al-Qaeda attack on the United States.
Their warning comes as a taped message purported to be from Osama Bin Laden's right-hand man vowed revenge on Washington if it harmed any Muslims held prisoner by the US in its ongoing war on terror.
The US is still checking whether the voice on the tape is indeed that of senior al-Qaeda leader Ayman al-Zawahri.
But the US Attorney General, John Ashcroft, says the key message in the recording is that al-Qaeda is still bent on injuring the US.
Mr Ashcroft and the Homeland Security Secretary Tom Ridge say that anti-terrorism measures introduced since the 11 September 2001 attacks have prevented more than 100 new strikes.
'Matter of time'
But Mr Ridge says there is still a high likelihood of a further attack.
"I feel everyday that we operate towards the notion that there will be one," he said.
"I think we have to and I don't think that the horizon that we look at should be anything other than accepting the reality that we are a target.
"Everything we stand for is anathema to all of these people who would do us harm."
Tom Ridge's homeland security department has taken further measures over the weekend to protect the US, by stopping foreigners without visas en route to other countries from passing through the US.
While such moves may stop some assailants, the administration's view seems to be that it is only a matter of time before some attack succeeds.