America's top intelligence officials have delivered a sobering assessment to Congress, saying that the al-Qaida terrorist network remains a persistent threat to the United States because its followers have been able to adapt their methods to make detection difficult.

National Intelligence Director Dennis Blair testified Wednesday to the House of Representatives' Permanent Select Committee on Intelligence, that the threat from al-Qaida remains strong.

"We have been warning in the past several years that al-Qaida itself, and its affiliates and al-Qaida-inspired terrorists remain committed to striking the United States. And in the past year, we have some names that go behind these warnings," he said.

Blair named as examples from the past year Najibullah Zazi, an Afghan-born man charged with plotting to use weapons of mass destruction in the United States and Umar Farouk Abdulmutallab, a Nigerian man who allegedly tried to blow up a U.S.-bound airliner on Christmas Day.

Blair also cited Major Nidal Hasan, an American who allegedly shot and killed 13 people at Fort Hood, in Texas, as an example of a "homegrown, self-radicalized extremist."

Blair warned that the threat of violent extremism is constantly evolving.

"We have made complex, multi-team attacks very difficult for al-Qaida to pull off," he said. "But as we saw with the recent rash of attacks last year, both successful and unsuccessful, identifying individual terrorists, small groups with short histories, using simple attack methods, is a much more difficult task," said Blair.

At a similar hearing held by the Senate Select Intelligence Committee on Tuesday, Central Intelligence Agency Director Leon Panetta said that it is not so much that the United States faces an attack like the September 11, 2001 terrorist attack.

"I think the greater threat is that al-Qaida is adapting their methods in ways that often times make it difficult to detect," he said.

Panetta and other intelligence officials warn that al-Qaida is increasingly trying to recruit Americans and others with no terrorist records, and that the danger of self-radicalization by terrorist sites on the Internet is growing.

Senate Intelligence Committee Chairwoman Dianne Feinstein asked Blair about the possibility of an attack against the United States in the near future.

FEINSTEIN: "What is the likelihood of another terrorist attempted attack on the U.S. homeland in the next three to six months, high or low? Director Blair?" BLAIR: "An attempted attack the priority is certain, I would say."

Director Blair cited malicious cyber activity as another increasing threat. But he said that threat of global economic collapse has decreased substantially since last year.

At Tuesday's Senate hearing, Republican lawmakers sharply criticized the Obama administration for its handling of the foiled Christmas Day airline bomb plot, complaining that suspect Abdulmutallab stopped talking after he was given a lawyer.

National Intelligence Director Blair said authorities have to strike a balance between the goals of getting good intelligence and prosecuting the suspect, and that they had done a good job in the case.

"We got good intelligence, we are getting more," Blair said.

Obama administration officials say that Abdulmutallab has been cooperating with the Federal Bureau of Investigation and is now sharing information.

A federal grand jury has indicted the 23-year-old on six criminal counts, including attempted use of a weapon of mass destruction and attempted murder. He has pleaded not guilty.

The case has sparked a heated debate in Congress over whether terror suspects should be taken into civilian or military custody, and tried in civilian courts in the United States or by military tribunals in Guantanamo Bay, Cuba.

tribunal: a type of court with the authority to deal with a particular problem or disagreement 特別法庭；裁判所

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（來源：VOA 編輯：陳丹妮）