Washington ? The U.S. government has arrested an American citizen accused of conspiring with al-Qaida terrorists to build and detonate a radioactive “dirty” bomb in this country, possibly in the nation’s capital.

Attorney General John Ashcroft said that Abdullah Al Mujahir, a former Chicago street gang member who also goes by the name of Jose Padilla, was in the custody of the U.S. military and was being treated as an enemy combatant. This suggests plans for the first military tribunal of an alleged terrorist since the World Trade Center in New York and the Pentagon were struck Sept. 11 by hijacked commercial airliners.

The attorney general, who was in Moscow on other business, made the announcement through a television hookup. Ashcroft said that Mujahir, who converted to Islam, was arrested May 8 as he flew from Pakistan into Chicago’s O’Hare International Airport. The 31-year-old is a native of Brooklyn, N.Y., who moved to Chicago at age 4.

“We have disrupted an unfolding terrorist plot to attack the United States by exploding a radioactive dirty bomb,” he said, adding that the government’s suspicions about Mujahir’s plans came from “multiple, independent, corroborating sources.” 1/4

Asked at a news conference here whether authorities had identified any co-conspirators in the United States, Deputy Attorney General Larry Thompson said, “We’re not going to comment on that.”

FBI Director Robert Mueller said, “Our principal interest is in preventing future terrorist attacks. This instance is an example of prevention.”

A senior administration official speaking on condition of anonymity said Mujahir was trained by al-Qaida in Afghanistan and Pakistan to wire explosives and to research radioactive dispersal devices. He was not believed to have had a bomb at the time of his apprehension.

“We don’t believe it went beyond the planning stages,” the official said.

Deputy Secretary of Defense Paul Wolfowitz, appearing at a news conference with Thompson and Mueller, said officials could not say with certainty that the nation’s capital was the likely target, although he said that Mujahir “did indicate knowledge of the Washington, D.C. area.”

Mujahir was taken Monday morning to a high-security U.S. Navy brig in Charleston, S.C., said Pentagon spokesman Lt. Col. Rivers Johnson, who said Mujahir was transferred from Justice Department custody in New York City.

Military officials have not decided whether to charge Mujahir or what charges to file, the military spokesman said.

Mujahir had a lawyer in New York but his access to a lawyer probably will be severely restricted now that he is in military custody, Johnson said. He said the alleged al-Qaida operative was being held separately from other prisoners at the brig.

Ashcroft said Mujahir had served prison time in the United States in the early 1990s, then traveled to Afghanistan and Pakistan during 2001 and met with al-Qaida officials. Ashcroft said Mujahir “trained with the enemy, including studying how to wire explosive devices and researching radiological dispersion devices.”

Ashcroft said al-Qaida apparently believed that Mujahir would be permitted to travel freely within the United States because of his U.S. citizenship and because he carried a U.S. passport.

The probable target of Mujahir’s plans to detonate the bomb was Washington, according to a U.S. official, speaking on condition of anonymity.

Another government official, who spoke only on condition of anonymity, said the intelligence that led to Mujahir’s arrest came from captured al Qaida leader Abu Zubaydah during recent interrogations.

This official said Mujahir is a former Chicago street gang member who converted to Islam after serving time in the United States, and met with an al-Qaida leader in 2001, before returning to the United States.

Said Ashcroft: “We have acted with legal authority both under the laws of war and clear Supreme Court precedent, which establishes that the military may detain a United States citizen who has joined the enemy and has entered our country to carry out hostile acts.”

Mujahir discussed several terrorist plans with Abu Zubaydah, the bin Laden lieutenant now in U.S. custody, according to a U.S. official.

Mujahir first met with Abu Zubaydah in Afghanistan in 2001, and traveled to Pakistan at Abu Zubaydah’s request, the official said, adding that he was one of a group that traveled with Abu Zubaydah to several locations in Pakistan.

Mujahir and another unidentified associate researched dirty bombs in Lahore, Pakistan, the official said.

“The radiological device plan articulated by (Mujahir) Padilla and his associate was in the planning stages, and no specific time was set to occur,” the official said.

At Abu Zubaydah’s behest, he also traveled to Karachi, Pakistan, to meet with several senior al-Qaida operatives, to discuss the plan, the official said. Mujahir also was interested in plans to bomb hotel rooms and gas stations in the United States, the official said.

It was unclear whether any of these meetings took place after Sept. 11.

President Bush, based on recommendations from Ashcroft and White House councel Al Gonzales, designated the suspect as a combatant in papers signed late Sunday. That designation allowed the Department of Defense to take custody of Mujahir from the Department of Justice.

“Based on the facts in this case and the importance of protecting sources who helped us get him, the determination was made that DOD is best for his detention,” an official said, speaking on condition of anonymity. This official said the administration does not know how close the suspect was to obtaining a so-called “dirty bomb.”