U.S. officials have announced a series of steps to shore up security at borders, airports and seaports, bracing for terrorist attacks in the event of war with Iraq.
The terror alert level now stands at orange, signifying a high risk of terrorist attack.
Homeland Security chief Tom Ridge has ordered heightened security at U.S. borders, airports and seaports, anticipating a greater terrorist threat as the United States moves closer to war with Iraq.
Security is also being tightened at chemical and nuclear plants and state governors have been asked to deploy National Guard troops as necessary to help local police.
Mr. Ridge says the terror threats could come in many forms.
"That while al-Qaida and those sympathetic to their cause are still the principal threat, Iraqi state agents, Iraqi surrogate groups, other regional extremist organizations and ad hoc groups or disgruntled individuals may use this time period to conduct terrorist attacks against the United States and our interests, either here or abroad," he said.
Counter-terrorism experts say a U.S.-led invasion of Iraq could give any number of terrorists-in-waiting a reason to attack American targets.
If a terrorist group can pull off an attack here, they will certainly try it," said Richard Clarke, a former National Security Council official who worked on counter-terrorism issues in both the current Bush administration and the Clinton administration.
He told ABC's This Week program that it may not be possible for the FBI and other law enforcement groups to monitor everyone suspected of having ties to terrorist groups.
"By definition, we don't know about [terrorist] sleeper cells," he said. "The FBI is following some people now in the United States, doing surveillance of them now. But there could be groups that we don't know about."
The FBI plans to mobilize as many as 5,000 agents to guard against terrorist attacks if there is a war with Iraq and keep tabs on thousands of Iraqis living in the United States.
Some terrorism experts believe the United States is close to dismantling the leadership of al-Qaida, thanks in large part to the recent arrest in Pakistan of operations chief Khalid Shaikh Mohammed.
But former National Security Council official Richard Clarke says al-Qaida has been adept at what he calls "re-growing leaders." He also says he is dubious about the notion that the recent arrests have dealt al-Qaida a lethal blow:
"Tens of thousands of people were moved through the al-Qaida training camps in Afghanistan over the course of the last decade," he said. "They are spread all over the world and there will be new recruits. So we haven't seen the last of al-Qaida."
Another terrorism expert, Neil Livingstone, believes al-Qaida is still dangerous because it has something to prove to the world in the wake of the recent arrests.
"I think al-Qaida is going to be desperate right now to demonstrate that while they have been severely wounded with this capture that they are not out of business," he said. "And so they will be attempting to carry out some attack somewhere to say look, we are still viable, we are still there and we still have to be taken seriously."
President Bush addressed the terror threat in his speech Monday, warning that the United States will not be intimidated by "thugs and killers."