The arrest Monday of a man allegedly plotting to build and explode a “dirty” radioactive bomb was somewhat like the presumed impact of such a device – a stunning moment followed by aftereffects yet to be revealed.

The stunning moment confirmed the worst post-Sept. 11 speculation. Al-Qaida has more attacks planned within the United States. They will be carried out not by foreigners but by Americans recruited to terrorism’s cause. They may employ chemical, biological or – the recurring cultural nightmare of the last half-century – nuclear agents.

There have, of course, been other al-Qaida attacks elsewhere and other attempts. A truck bomb in Tunisia in April that killed 19, a bomb in May in Pakistan that killed 11, the foiled attempt by Richard Reid in December all have been linked to Osama bin Laden’s organization. The accused in this case, Abdullah al Muhajir, was born in Brooklyn and raised in Chicago as Jose Padilla. His background is that of a common criminal, not a religious fanatic; had he not been so clumsy in disguising his al-Qaida connections and his interest in obtaining material for a dirty bomb, his profile, though unsavory, is not one that would have set off anti-terrorism alarms. Experts have tried to reassure the public that a small amount of stolen radioactive waste (from medical or industrial sources, most likely) packed in with common explosives would not cause widespread deaths, but the thought of an American city being turned into Chernobyl is horrifying.

Some aftereffects will be political. Certainly, this arrest reminds the public that a war is still being waged, it no doubt will strengthen support for the president’s conduct of that war and, after months of vague and unsettling terrorism alerts, it should restore some confidence in the ability of the FBI and CIA to work together effectively. Because al Mr. Muhajir has been held in federal custody as a material witness for a month before he was arrested and transferred to military custody, there will be questions about whether this bombshell was timed to blunt the congressional inquiry into the security lapses before Sept. 11.

No one should expect a definitive answer to this question, but the one that must be answered concerns the societal aftereffect of this blast: What rules, what laws, protect the rights of American citizens who might find themselves suddenly declared soldiers for the enemy?

As a material witness, Mr. Muhajir had the oversight of a federal judge and access to counsel. As an enemy detainee in military custody of the military, he has neither and can be held for any length of time – forever, in fact – without any evidence being presented against him. He may well be a very dangerous person who, given the chance, would have poisoned a city. But he is also an American citizen and this truncation of his rights occurred without a court hearing, but instead by administration fiat.

If, as this arrest indicates, al-Qaida’s strategy now is to enlist Americans to wage war upon their own, this question is one that all three branches of government must work together to answer.