Osama bin Laden’s al-Qaida terror network continues to pose a serious threat, as shown by the arrest of an American suspected of preparing a ‘‘dirty’’ bomb attack on the US, Attorney General John Ashcroft said today.

The US Attorney General today said the American arrested on suspicion of plotting to detonate a ‘dirty’ radiological bomb in the US was a key al-Qaida operative who has ‘‘very significant information’’ about the terror network.