(CBS) U.S. officials are backing away from assertions that a man arrested last month in Chicago was plotting a 'dirty' bomb attack on the United States, CBS News Correspondent Jim Stewart reports.

U.S. officials now admit they're not sure what American-born Abdullah al Muhajir's plans were when he returned to the U.S. last month. And any plot, including possibly exploding a radiation bomb in Washington D.C., was in the "initial planning stages." But given what he'd trained for in Afghanistan, they expected the worst.

"He researched nuclear weapons and received training in wiring explosives while in Pakistan, and he was instructed to return to the United States to conduct reconnaissance operations for al Qaeda," said Deputy Defense Secretary Paul Wolfowitz.

Investigators had known of Muhajir for some time, including his alias. Much of their information about him came from Abu Zubaydah – al Qaeda's top surviving operations officer until his capture last March – who tipped them off to Muhajir's American mission.

"He had indicated some knowledge of Washington, D.C., but I want to emphasize again that there was not an actual plan. We stopped this man in the initial planning stages," said Wolfowitz. That was echoed by FBI Director Robert Mueller, who also said the plan to explode a dirty bomb had not gotten past the planning stages.

Muhajir, born in New York as Jose Padilla, was detained by the FBI in Chicago over a month ago, when he flew in from Pakistan on May 8. Authorities descirbed him as a former Chicaco gang member who was raised Catholic but converted to Islam.

Commenting on Tuesday, Attorney General John Ashcroft said the suspect was involved in "very serious terrorist plots."

Speaking from Hungary, the latest stop on his overseas trip to discuss worldwide anti-terrorism efforts, Ashcroft described al Muhajir as a key al Qaeda operative who has "very significant information" on other terror suspects.

Al Mujahir was taken from Justice Department custody in New York City Monday morning to a high-security U.S. Navy brig in Charleston, S.C., said Pentagon spokesman Lt. Col. Rivers Johnson. Military officials have not decided whether to charge Muhajir or what charges to file, Johnson said.

Muhajir 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. The alleged al Qaeda operative is being held separately from other prisoners at the brig, Johnson said.

Officials said al Muhajir, a 33-year-old American, traveled to Pakistan and Afghanistan in 2001 and met with senior al Qaeda officials to discuss the plan.

The officials would not say whether the meetings took place before or after the Sept. 11 attacks on New York and Washington that killed 3,000 people and which the United States has blamed on Saudi-born dissident Osama bin Laden and his al Qaeda network.

Another official, who spoke on condition of anonymity, said al Muhajir was "probably" targeting the U.S. capital.

U.S. officials knew al Muhajir was on the airplane heading into Chicago O'Hare International Airport on May 8, where he was detained upon arrival.

Mueller, whose bureau has been under fire for failing to share information with other U.S. intelligence agencies that might have prevented the hijacked airliner attacks, said the CIA played a key role leading up to the arrest.

After a 1991 arrest in South Florida on a handgun-related charges, Padilla began referring to himself as Abdullah al Muhajir, officials said. He lived in South Florida's Broward County from 1990 to 1997 and was convicted of aggravated assault with a firearm in Sunrise in 1992.

When he was arrested, he told police he had worked at a Holiday Inn in Plantation setting up banquet rooms for two weeks. Records show he has his name "Jose'' tattooed on his right arm.

While in the Broward County Jail, he was accused of battery on a jail officer and resisting without violence in January 1992.

Al Muhajir, who has family in the United States, was last in the United States in 1998. After that he traveled mostly in the Middle East, officials said.

"While in Afghanistan and Pakistan, al Muhajir trained with the enemy, including studying how to wire explosive devices and researching radiological dispersion devices," Ashcroft said.

"Al Qaeda officials knew that as a U.S. citizen, holding a valid U.S. passport, al Muhajir would be able to travel freely in the United States without drawing attention to himself."

Al Muhajir is being held by the Defense Department as an "enemy combatant," which under the rules of war allows him to be questioned without the usual protections afforded in the U.S. judicial system, like having an attorney present. Officials have not ruled out lodging criminal charges against al Muhajir later.

President Bush has said American citizens would not be tried in military tribunals that were created after Sept. 11 to try foreign terrorists outside the U.S. court system.

Al Muhajir becomes the third U.S. citizen detained since the Sept. 11 attacks. John Walker Lindh, 21, who was arrested in Afghanistan, faces charges in U.S. court in Virginia of conspiring to murder Americans and providing services to the Taliban and al Qaeda. The other is Yasser Essam Hamdi, 22, an American-born prisoner who was transferred from Guantanamo to a prison at Norfolk Naval Station in Virginia.