Pakistan nuclear scientist held

American investigators are questioning an eminent Pakistani scientist over intelligence reports that indicate Osama bin Laden's Al Qaeda terrorist network has acquired the technology to manufacture a viable nuclear bomb.

In what is being seen as one of the most disturbing developments in the unfolding inquiry into Al Qaeda, Dr Sultan Bashiruddin Mehmood was arrested by Pakistani secret servicemen at his home in Lahore and interrogated by a team of FBI and CIA agents.

Dr Mehmood was a leading figure in Pakistan's Atomic Energy Commission until his resignation over disagreements with government policy. He is a staunch supporter of Afghanistan's Taliban regime and has helped establish a number of industrial facilities in the country using mainly Islamic funds.

It is understood he has been closely questioned about a complex of buildings outside Kandahar that have been described as a flour mill. He was involved in establishing the factory through an organisation called Ummah Reconstruction. The buildings were targeted in recent allied bombing attacks but a shortage of good intelligence from inside Afghanistan has made it impossible to determine precisely what was inside or whether items had recently been removed.

However, according to a number of intelligence reports passed to American investigators, Al Qaeda has already acquired the material and the knowledge to put together a so-called "dirty bomb". Such a device uses conventional explosives, around which radioactive material is packed.

When the bomb is detonated radioactive particles are blasted over a wide area. The effects could wipe out large sections of a city and render it uninhabitable.

Until recently it was believed that Bin Laden had failed in a number of attempts to buy nuclear material. Earlier this year, a Pakistani who described himself as a chemical engineer tried to acquire nuclear material from a Bulgarian middleman.

There was convincing evidence that the Pakistani was acting for Bin Laden's network. The deal fell through.

Then, a few months ago, the authorities in Uzbekistan intercepted 10 leadlined containers of radioactive material bound for Quetta, Pakistan - a border town with Afghanistan. This has led investigators to believe that some radioactive material has reached Afghanistan and Bin Laden.

Inquiries are continuing but the arrest has focused renewed attention on the allies' greatest fear, that Bin Laden will unleash a weapon of mass destruction on the US or Britain.