Washington ? The possibility that al-Qaida or its sympathizers could gain access to a nuclear bomb is the greatest danger facing the United States in the war on terrorism, Atty. Gen. John Ashcroft said Thursday.

U.S. officials “from time to time” uncover evidence terrorists are trying to develop nuclear capability, Ashcroft said without providing any specifics. It is not clear whether they have made any progress, but the United States must take the threat seriously, he said in an interview.

Ashcroft, 62, is ending four years as the nation’s chief law enforcement officer. He will leave office when his successor, Alberto Gonzales, is confirmed by the Senate and sworn in, possibly next week.

Since the 9-11 attacks, Ashcroft has been vilified for pushing controversial counterterrorism policies, which critics say undermine freedoms.

They include the Patriot Act, which bolstered FBI surveillance and law enforcement.

Ashcroft made no apology for his actions, saying he has enjoyed full support from President Bush.

His greatest failure, Ashcroft said, was in not fully explaining to the American people early on just how the Patriot Act has helped in that war. Time will prove that the law has not been the threat to the Constitution seen by some, he said.

“Rights have not been infringed. Human dignity has not suffered. It’s been enhanced and it has not carried a cost or toll on the civil liberties of America,” Ashcroft said.