Attorney General John Ashcroft on Monday said United States authorities had captured an American Al Qaeda operative and prevented an attack on the US with a radioactive dirty bomb.‘‘I can tell you we have a man detained who is a threat to the country, and that thanks to the vigilance of our intelligence-gathering and law enforcement, he is now off the streets, where he should be,’’ US President George W. Bush told reporters as he met Israeli Prime Minister Ariel Sharon. US talks ‘pre-emptive’ strikes Washington: The Bush administration is developing a new doctrine that moves away from the Cold War pillars of containment towards a policy that supports pre-emptive attacks against terrorists and hostile states with chemical, biological or nuclear weapons. The new doctrine will be laid out by President Bush’s National Security Council as part of the ‘‘National Security Strategy’’ being drafted for release this fall. It will add ‘‘pre-emption’’ and ‘‘defensive intervention’’ as formal options for striking at nations. LATWP ‘‘We have captured a known terrorist who was exploring a plan to build and explode a radiological dispersion device, or dirty bomb in the US,’’ Ashcroft said in a televised announcement from Moscow.Ashcroft added Abdullah al Mujahir, also known as Jose Padilla, was detained more than a month ago, on May 8, after flying into the Chicago O’Hare International Airport from Pakistan. ‘‘On apprehending al Mujahir as he sought entry into the US, we have disrupted an unfolding terrorist plot to attack America by exploding a radioactive dirty bomb,’’ Ashcroft said.A so-called dirty bomb involves exploding a conventional device wrapped in radioactive material that can kill victims in the immediate area and spread highly toxic material to humans, causing mass death and injury.Ashcroft, in Moscow for meetings with Russian officials, said the US Government had ‘‘multiple, independent and corroborating sources’’ that al Mujahir was closely associated with Al Qaeda — the militant network US holds responsible for the September 11 hijacking attacks in New York that killed 3,000 people — and was ‘‘involved in planning future terrorist attacks on innocent American civilians in the US’’.Following his release from prison, al Mujahir traveled to Afghanistan and Pakistan and on several occasions in 2001, he met with senior Al Qaeda officials, Ashcroft said. ‘‘While in Afghanistan and Pakistan, al Mujahir trained with the enemy, including studying how to wire explosive devices and researching radiological dispersion devices,’’ he said, adding, ‘‘Al Qaeda officials knew that as a US citizen, holding a valid US passport, al Mujahir would be able to travel freely in the US without drawing attention to himself.’’Al Muhajir was being held by the Defense Department as an ‘‘enemy combatant’’ despite the fact he is a US citizen.