Al Qaida can be expected to attempt an attack on America in the next three to six months, intelligence chiefs told the US Congress.
The terror organisation was deploying operatives to the US to carry out new attacks from inside the country, including "clean" recruits with no trail of terrorist contacts, CIA director Leon Panetta said.
The chilling warning comes it emerged Christmas Day airline attack suspect Umar Farouk Abdulmutallab was co-operating with investigators.
Al Qaida is also inspiring homegrown extremists to trigger violence on their own, Mr Panetta said.
The annual assessment of the nation's terror threats provided no startling new terror trends, but increased growing concerns since the airline attack in Detroit that militants are growing harder to detect and moving more quickly in their plots.
"The biggest threat is not so much that we face an attack like 9/11. It is that al Qaida is adapting its methods in ways that oftentimes make it difficult to detect," Mr Panetta told the Senate Intelligence Committee.
Several senators clashed over whether suspected terrorists should be tried in civilian or military court.
At the same time, a group of bi-partisan politicians introduced legislation that would force the Obama administration to backtrack on its plans to try September 11 defendants in federal court in New York and use military tribunals instead.
Mr Panetta said as al Qaida planned new terror plots, it was increasingly relying on new recruits with minimal training and simple devices to carry out attacks and also warned of the danger of extremists acting alone.
"It's the lone-wolf strategy that I think we have to pay attention to as the main threat to this country," he said.
The hearing comes just over a month since the failed attempt to bring down a passenger plane in Detroit by Nigerian Abdulmutallab, a former London student.
And the assessment comes only a few months after US Army major Nidal Hassan is accused of single-handedly attacking his fellow soldiers at Fort Hood, Texas, killing 13.
National intelligence director Dennis Blair said thanks to changes made since the December 25 attack, US intelligence would he able to identify and stop someone like the Detroit bomber before he got on the plane. But he warned a more careful and skilled would-be terrorist might not be detected.
FBI director Robert Mueller defended the bureau's handling of the Detroit attack, disputing claims that agents short-circuited more intelligence insights from Abdulmutallab by quickly providing him with his Miranda rights to remain silent.
Mr Mueller said that in "case after case" terrorists had provided actionable intelligence even after they were given their rights and charged with crimes because they knew such co-operation could mean shorter sentences or other consideration from the government.
Hundreds of terror suspects have already been convicted in civilian courts, including convicted British shoe bomber Richard Reid.
But Republican senator Lindsey Graham offered a Bill yesterday that would ban the government from using Justice Department funds to prosecute suspects charged in the September 11 attack in civilian courts.
The move comes on the heels of the Obama administration's decision to rethink whether it would try 9/11 mastermind Khalid Sheikh Mohammad in a New York City court.
The proposed law would cover people who legally could be prosecuted by a military commission, applying to terror suspects who are not US citizens. By early today, the Bill had support from 18 senators, mostly Republicans.
During the terror assessment hearing, Mr Blair also warned of the growing cyberthreat, saying computer-related attacks had become dynamic and malicious.
The government's first quadrennial homeland security review says high consequence and large-scale cyberattacks could massively disable or hurt international financial, commercial and physical infrastructure.
The report, obtained by The Associated Press, said these types of cyberattacks could cripple the movement of people and goods around the world and bring vital social and economic programmes to a halt.