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WASHINGTON (AP) _ The U.S. government has arrested an alleged al Qaida terrorist who plotted to build and detonate a radiological ``dirty'' bomb, Attorney General John Ashcroft said Monday.

Ashcroft said Abdullah Al Mujahir was in the custody of the U.S. military and being treated as an enemy combatant, suggesting plans for the first military tribunal of an alleged terrorist.

The attorney general, who was in Moscow, made the announcement through a television hookup.

Ashcroft said Al Mujahir, also known as Jose Padilla, was arrested May 8 as he flew from Pakistan into Chicago O'Hare International Airport.

``We have disrupted an unfolding terrorist plot to attack the United States by exploding a radioactive dirty bomb,'' Ashcroft said.