An explosive device in the midst of a turf war

Just as the threat of nuclear war between India and Pakistan was starting to recede, the US Attorney General, John Ashcroft, lobbed a new incendiary device into the cauldron of international war fever. The US, he said, had arrested a "known terrorist" trying to enter the country through Chicago, thereby preventing an attack on America with a radioactive "dirty bomb".

Now "dirty bombs" are not to be trifled with. But before the cheering for heroes of US law enforcement drowns out all other sounds, it is worth looking closely at what Mr Ashcroft said, why he may have said it, and why now.

According to the attorney general, Abdullah al-Mujahir, also known as José Padilla, was detained after arriving in the US from Pakistan. He was, said Mr Ashcroft, "exploring a plan to build and explode a radiological dispersion device, or dirty bomb, in the US". He added that the US had "multiple, independent and corroborating sources" that Mr al-Mujahir was associated with al-Qa'ida and "involved in planning future terrorist attacks on innocent American civilians".

The impression left by Mr Ashcroft was that American civilians were in imminent danger from one of the most deadly combinations of dangerous materials known to man. And, nine months since the terrorist attacks on the US, the administration is surely justified in taking every precaution to avert further atrocities. Mr al-Mujahir may indeed have presented such a threat.

Yet the US track record of identifying "known terrorists" is far from reproachless – as its flawed extradition requests to Britain, not to mention the hundreds of suspects still detained without charge at Guantanamo Bay, show. And it is hard to separate the timing of yesterday's dread disclosure from the rather more prosaic, if less physically threatening, turf wars raging between the CIA and the FBI over who knew what, and when, before 11 September. President Bush meanwhile has his back to the wall over charges that he was briefed on specific terrorist dangers back in August.

No one doubts "dirty bombs" are lethal and should on no account be used – but neither should they be misused.