Washington ? The government on Monday announced the arrest of an American accused of plotting with al-Qaida terrorists to detonate a “dirty bomb” to spread radioactive material, possibly targeting Washington.

Authorities said the alleged scheme, involving a former gang member from Chicago who was raised Catholic but converted to Islam, 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.

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.

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.

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. It has been called an ideal terrorist weapon.

Atty. Gen. John Ashcroft said al-Qaida apparently believed Padilla would be permitted to travel freely within the United States because of his citizenship and his U.S. passport.

In a statement attributed to al-Qaida 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 said Padilla met several times in 2001 with senior al-Qaida 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-Qaida leader Abu Zubaydah, one of Osama bin Laden’s top lieutenants, said two U.S. officials.