The American accused of plotting with Al Qaeda terrorists to detonate a "dirty bomb" to spread radioactive material, possibly targeting Washington, had at least one accomplice now detained by authorities, Fox News has learned.

Law enforcement sources told Fox News another man named Benjamin Ahmed Mohammed was implicated in the plot and was taken into custody in Pakistan "recently," perhaps late last month. One official said he would continue to be detained in Pakistan and there are currently no plans to bring him to the United States.

The primary suspect, Jose Padilla, 31, also known as Abdullah al Muhajir, was arrested on May 8 as he flew from Pakistan via Zurich, Switzerland, to O'Hare International Airport. Officials said the CIA and FBI had helped foil the alleged plan, and FBI agents were waiting for Padilla as his plane arrived at the gate. Authorities said they believed he had returned to conduct reconnaissance for Al Qaeda.

A government source, speaking on condition of anonymity, said Padilla and at least two others who may have been involved in the alleged plot were detained in Pakistan on immigration violations before May 8. But Padilla was allowed to board his international flight and tricked into believing he had escaped — with U.S. agents sitting on the plane quietly watching his every move.

Padilla is a former gang member from Chicago who was raised Catholic but converted to Islam. Authorities said the alleged scheme went only as far as the planning stages. Undersecretary of State John Bolton indicated the man was carrying plans for the attack when he was picked up in Chicago.

President Bush said, "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."

A "dirty bomb" — traditional explosives combined with radioactive material — would not result in a nuclear explosion, but a powerful device could release small amounts of radioactive material over dozens of city blocks. Experts believe the most devastating effect would be the panic caused and the difficulty sending rescue workers into the contaminated area. For that reason, it has been called an ideal terrorist weapon.

In an unusual legal twist, the Justice Department handed the Brooklyn-born Padilla to the Pentagon for indefinite imprisonment as an "enemy combatant." Government lawyers cited a 1942 Supreme Court ruling permitting such a transfer. Padilla had been held quietly for weeks in New York, then was flown Monday aboard a military C-130 plane to a Navy brig in Charleston, S.C.

Attorney General John Ashcroft said Al Qaeda apparently believed Padilla would be permitted to travel freely within the United States because of his citizenship and his U.S. passport.

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

A U.S. official, speaking on condition of anonymity, said the apparent target was Washington. Speaking at a news conference, Deputy Defense Secretary Paul Wolfowitz said the scheme was "still in the initial planning stage," and that Padilla "had indicated some knowledge of the Washington, D.C., area."

Another U.S. official said Washington was believed one possible target because of its prominence as the seat of government, not because of any firm evidence from Al Qaeda.

White House spokesman Ari Fleischer said investigators do not believe Al Qaeda has acquired enough radioactive material to build such a weapon.

In a statement attributed to Al Qaeda spokesman Sulaiman Abu Ghaith, the group said: "We have the right to fight (Americans) by chemical and biological weapons so that they catch the fatal and unusual diseases that Muslims have caught due to their chemical and biological weapons."

Ashcroft, who first disclosed the arrest in a television announcement from Moscow, said Padilla "trained with the enemy," studying how to wire explosives and researching radiological weapons. Ashcroft said Padilla met several times in 2001 with senior Al Qaeda officials in Pakistan and Afghanistan, where he traveled after he served one year's probation on state weapons and assault charges in Sunrise, Fla.

Information leading to Padilla's arrest came in part from U.S. questioning of captured Al Qaeda leader Abu Zubaydah, one of Usama bin Laden's top lieutenants, said two U.S. officials. Ashcroft said information about the plot came from "multiple independent and corroborating sources."

Padilla first met Zubaydah in Afghanistan in late 2001 after the Sept. 11 terror attacks, then went to Lahore, Pakistan, to research dirty bomb techniques with an unidentified associate, officials said. At Abu Zubaydah's request, Padilla traveled to Karachi, Pakistan, in March to meet several senior Al Qaeda officials and discuss bombings of U.S. gas stations and hotels, officials said.

Bolton, the State Department undersecretary, referred to "the arrest of the terrorist and the plans that he was captured carrying." Bolton declined to elaborate on his remarks.

Padilla was described by one former neighbor in Chicago as "so quiet, so nice," nicknamed "Pucho" as a teen-ager and who enjoyed basketball and video games with his friends.

"He doesn't look like a person who would do something like that," said Nelly Ojeda, 64, who lived in the same three-story flat as Padilla.

But Padilla's criminal history in the United States dates even to those days growing up in Chicago, where his family moved when he was 4. He was convicted at 15 as a juvenile of aggravated battery, armed robbery and attempted armed robbery. A law enforcement official said Padilla was in custody there between November 1985 and May 1988.

In Florida, he was convicted in 1991 in Sunrise on charges of aggravated assault and discharging a firearm, court records show. Padilla, who identified himself as Catholic when he was booked on those charges, served one year of supervised release, until Aug. 4, 1993. A Justice official, speaking on condition of anonymity, said Padilla converted to Islam after his 1993 release.

Police said Padilla — who has "Jose" tattooed on his right arm — brandished a .38 revolver at another driver during a traffic encounter. When the other driver followed him to a gas station, Padilla fired one shot from his vehicle at the other car. No one was hurt. Police traced his license tag and arrested Padilla at home, where they found him with the handgun in his waistband.

While in the Broward County jail, Padilla was accused of battery on a jail officer and resisting without violence in January 1992. He pleaded guilty and spent 10 months behind bars.

Court records also show Padilla with speeding convictions in 1993 and 1997, and his driver's license was suspended in 1996. Padilla also completed a traffic law substance-abuse course in 1992, but it was unclear whether that stemmed from an arrest.

Padilla was assigned a lawyer in New York immediately after his May 8 arrest, but his access to his lawyer probably will be severely restricted now that he is in military custody, said Lt. Col. Rivers Johnson, a Pentagon spokesman. Officials said there were no plans to organize a military tribunal or otherwise pursue criminal charges against Padilla, in part because tribunals are reserved for accused terrorists who are not U.S. citizens.

Padilla 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.

The Associated Press contributed to this report.