Last August a Chechen black widow blew herself up outside the Rijskaya metro station leaving 10 dead.London's tragedy will therefore strike a horrible chord in Russia. About 120 hostages and almost 50 Chechen extremists were killed when Russian special forces used a mysterious and powerful knock-out gas to recapture the theatre.That was preceded by the seizure in 1995 of a hospital in southern Russia along with 1,000 hostages. And to do so they will stop at nothing.Without question, Russia's worst terrorist atrocity to date was the Beslan school siege in southern Russia last September when armed gunmen and female suicide bombers took more than 1,000 people hostage.Some 330 people died, over half of them children, when the siege was finally broken and rebel gunmen traded fire and explosives with Russian troops.The Dubrovka theatre siege in Moscow in 2002 also left a terrible scar on Russia's public consciousness. Indeed, rebel Chechen websites are awash with Islamist extremist slogans.The Russian leader, Vladimir Putin, says Chechen extremists receive much of their funding from Saudi Arabia and has accused them of having close links with al-Qa'ida and Osama bin Laden himself.Indeed, the Russian Army and the FSB security service regularly claim to have "liquidated" foreign fighters and mercenaries in Chechnya, and there are consistent reports that Chechen fighters have attended training camps in the Middle East.According to the Kremlin, Basayev and his lieutenants want to set up an Islamist Caliphate in southern Russia, grouping together Chechnya and various other increasingly troubled regions. More moderate rebels say they merely want independence and "normal lives".In recent years, such moderates appear to have lost considerable ground, however and, earlier this year, Aslan Maskhadov, the rebel Chechen president considered a moderating influence by some, was killed when Russian special forces got their man.That left the stage to just one notorious Chechen warlord, the one-legged Shamil Basayev, Russia's most wanted man, who is on record as calling for a jihad or holy war to be waged across southern Russia.

Security was also stepped up on Amtrak, the national passenger rail network.At the Pentagon, where 168 people died when the American Airlines jet smashed into the home of the Defence Department, police officers were out in force in squad cars, on bicycles and on foot, on nearby highways and at the local Metro underground station.In Atlanta, all available commuter rail police officers and dogs were deployed. Meanwhile, Jose Padilla, a US citizen, has been held since 2002 on the suspicion that he was examining potential sites for a "dirty bomb" attack in Washington or another major US city.Nationwide, the US government's colour-coded threat level remains at the intermediate yellow level, in part because of widespread criticism that moving to a higher level causes unneccessary anxiety while providing few specifics.The last occasion the threat-level as at orange was in August last year, after intelligence that al-Qa'ida operatives were targeting buildings in the financial districts of New York, northern New Jersey and Washington DC. It returned to yellow the following November.In Washington, New York and Chicago, extra police, some of them with sniffer dogs, were visible, patrolling the cities' underground systems during the morning rush hour and beyond. They were consistent with known al-Qa'ida techniques, and similar to the Madrid bombings 16 months ago.In fact, despite many alarms and attacks elsewhere in Europe, Asia and Africa, the US has been terrorism-free since the attacks in New York and Washington almost four years ago.

Airport security has made a repeat of the 9/11 attacks all but impossible.Hundreds of people, many of them of Arab or Middle Eastern origin, have been detained, while others, despite strong domestic and international protest, have been handed over to other countries under the procedure known as "extraordinary rendition".Even so, yesterday's events in the capital that is Washington's closest ally in Iraq and the war on terror have increased fears of attacks in the US.Counter-terrorism officials point to the years of preparation that went into 9/11, and warn that al-Qa'ida sleeper cells may well be plotting new outrages.Although there are no specific indications of an increased terrorist menace to the US, security experts accept that further attacks in the US are all but inevitable. We will find them, we will bring them to justice," Mr Bush told reporters at Gleneagles, where he is attending the G8 summit. "The war on terror goes on."Last night, as a small team of FBI officials headed to London to help the investigation, US officials tried to determine the veracity of the claim by an al-Qa'ida cell that it was responsible for the London bombings, and were combing through the intelligence for clues that other attacks might be in the offing.But the initial assumption here was that Islamic extremist terrorism was involved. Officials add that in their next assault, terrorists might not use conventional explosives, as in Madrid or London, but chemical or biological agents, even a radioactive "dirty" weapon.The still unexplained mailings of anthrax spores to buildings on Capitol Hill in autumn 2001 gave Washington an unsettling taste of possible things to come. Those worries would only be heightened if it transpired that suicide bombers carried out any of the London attacks.The message then would be plain, that such a tactic could be just as easily be employed in Manhattan or Chicago as in Bloomsbury or the City of London. The fear now is that terrorists will shift from well-protected sites to soft targets, not only commuter trains and stations, but shopping malls, concerts and sporting events. But the government's colour-coded alert system is being raised to its second highest level of orange for the country's mass transit systems.Special attention is being focussed on five cities: Boston, New York, Washington DC, Atlanta and Miami.Mr Chertoff said: "When we look at what happened in London, and the al-Qa'ida tactic of taking simultaneous action in several places, prudence and common sense dictates we take these measures." But he assured Americans that the authorities had already put extra precautions in place for public transport systems across the country since the Madrid commuter train bombings of March 2004.Americans on the east coast learnt of the bomb attacks in London as they were preparing to go to work.