Police given extra time to hold 'dirty bomb' suspects

Anti-terrorist police were granted a further five days yesterday to question four men arrested after a Sunday newspaper claimed they were plotting to detonate a chemical bomb in Britain or the US.

The News of the World investigations editor, Mazher Mahmood, said the men had been searching for "red mercury", for which their alleged boss in Saudi Arabia was prepared to pay £300,000. On Friday, police arrested three men at a hotel in Brent Cross and the fourth at his home, also in north London. Searches of various addresses in and around London were not believed to have yielded suspicious material.

Under the headline "Blown Apart", the newspaper said it launched its investigation after it was tipped off that a "Mr Big" in Saudi Arabia - "a known al-Qa'ida hotbed" - was looking for materials for a dirty bomb. Mahmood said he went undercover as a "Muslim extremist" and in meetings in London made plans to supply up to a kilo of red mercury which he was told would be used to detonate a bomb in Britain or the US.

Red mercury is believed by some to have been made by the Russians in nuclear reactors during the Cold War by irradiating elemental mercury with antimony oxide. The result is claimed to be a reddish and potent radioactive material which can be used to make a "suitcase bomb". But the scientific community remains divided about whether such a mixture exists because none has been recovered.

The newspaper claimed that as Mahmood moved towards a deal, it called in officers from the Metropolitan Police SO13 anti-terrorist branch who made the arrests after a meeting at the Holiday Inn at Brent Cross. The newspaper said teams of surveillance officers and marksmen surrounded the hotel while a "convoy" of arresting officers waited in a nearby car park.

The expose is among hundreds masterminded by Mahmood who boasts that his investigations have led to about 100 convictions. His previous victims over the years have included Sophie, Countess of Wessex, Newcastle United directors Freddie Shepherd and Doug Hall and London's Burning star John Alford.

But media experts say his methods are sometimes questionable, and his track record was blemished in 2002 when the trial collapsed into the alleged plot to kidnap Victoria Beckham which Mahmood "exposed". Charges were dropped by the Crown Prosecution Service (CPS) after it learned that the newspaper had paid Florim Gashi, a convicted conman, £10,000 for his role in the sting.

The CPS decided that Gashi, who secretly filmed the men apparently discussing the plot, was not a reliable witness. Lawyers for the five accused men argued that transcripts of the tapes showed they discussed the kidnap idea only after Gashi suggested it.