Published: June 11, 2002

The Justice Department announced today that it had broken up a plot by Al Qaeda to detonate a radioactive bomb inside the United States by arresting an American citizen in the case.

''We have captured a known terrorist who was exploring a plan to build and explode a radiological dispersion device, or 'dirty bomb,' in the United States,'' Attorney General John Ashcroft said in a televised announcement from Moscow, where he was meeting with Russian official on unrelated matters.

Mr. Ashcroft identified the arrested man as Abdullah al-Muhajir, 31, a former Chicago gang member who American officials said was born Jose Padilla in Brooklyn and raised as a Roman Catholic but who converted to Islam and began using a new name.

Mr. Padilla has been in custody since May 8, when he was arrested on a sealed material witness warrant at O'Hare International Airport in Chicago as he arrived on a flight from Zurich.

Senior government officials said Mr. Padilla had discussed the bomb plot with top Qaeda leaders in Pakistan and Afghanistan, among them Abu Zubaydah, the Osama bin Laden lieutenant who was captured in Pakistan in March and later told United States officials about the plan. But they also said Mr. Padilla had not obtained the materials to make such a device.

Mr. Zubaydah, the most senior Qaeda leader in custody, told his American interrogators that several Qaeda members had come to him late last December with a proposal to acquire and detonate a radiological device, a so-called dirty bomb. Mr. Zubaydah did not identify Mr. Padilla by name, but provided enough information to allow the Central Intelligence Agency to check with other sources -- including documents seized in Afghanistan -- to narrow the search to Mr. Padilla, officials said.

''We were able to figure out who Zubaydah was talking about, and then screen him and follow him,'' said an American intelligence official.

In New York City, where Mr. Padilla was held after his arrest until being transferred on Sunday to a military jail in South Carolina, a law enforcement official described Mr. Padilla as someone who tried to make inroads with terrorists after his conversion to Islam.

Other officials said that before he left Pakistan, Mr. Padilla was told by Al Qaeda leaders to fly to the United States to conduct reconnaissance for several possible plots, including the possibility of blowing up hotel rooms and gas stations.

But the plot outlined by United States officials today centered on a plan to carry out an attack using a bomb that uses conventional explosives to spew potentially lethal radioactive material across a wide area.

American intelligence officials cautioned that the plot had been in early planning stages and no time for the operation had been set. They said that there was also no evidence that Mr. Padilla or any other Qaeda operatives had obtained the materials needed to construct a dirty bomb.

''They didn't seem to think they would have trouble getting radiological materials, but they didn't have any of it,'' said one official.

Donna Newman, Mr. Padilla's lawyer in New York, said federal authorities had given her little information about the accusations against Mr. Padilla. She also expressed dismay that the government had suddenly transferred him to the military jail in South Carolina.

American officials said Al Qaeda's leadership was apparently intrigued by Mr. Padilla's being an American citizen who might have an easier time of gaining entry to the United States than other Qaeda members.

The announcement of the arrest seemed to suggest that the Bush administration had succeeded in executing the kind of aggressive preventive action that officials say they have concentrated on since Sept. 11.