Police in London last night recovered part of the rocket launcher used in the attack on the headquarters of MI6, Britain's Secret Intelligence Service. Earlier they said the weapon used was of a type previously found on both sides of the Irish Border.
The discovery reinforced the widespread suspicion that the "Real IRA" was responsible, though the variety of weapon is also known to be available in countries such as Russia and the former Yugoslavia.
Londoners remained on terrorist alert last night as police continued their investigation into Wednesday night's missile attack. At the same time, Sussex Police announced a review of security measures for next week's Labour Party conference in direct response to the attack.
While dissident republicans remained the prime suspects, the head of Scotland Yard's antiterrorist branch, Mr Alan Fry, said he was keeping an open mind about the perpetrators and their motive and was not ruling out other potential terrorist involvement.
No one was injured in the attack which, British Foreign Secretary Mr Robin Cook said, had failed to disrupt the work of the service. "Minimal damage" was caused when the explosive device, thought to have been a grenade, exploded on the eighth floor of the building.
The attack brought early-morning chaos to London as police threw a 500-metre cordon around the building and began an inch-by-inch search for clues.
Waterloo station - one of London's busiest terminals, used by some 100,000 passengers daily - was closed until mid-morning, when Eurostar services resumed. It was not until teatime last night that police removed the cordon and reopened nearby Vauxhall station to the public.
Mr Fry confirmed there had been no warning of, and no claim of responsibility for, the attack, as he warned Londoners to remain vigilant to the possibility of further attacks. "We have a genuine threat of terrorism in London and that is against a number of targets."
Earlier, before confirmation of the discovery of part of the rocket launcher, Mr Fry said the device was fired at the building from a range of between 200 to 500 metres. "Clearly the sort of weapon we believe was used in this attack is known to be in the hands of certain groups. They will be uppermost in our minds," he said, adding: "Clearly we have to keep in mind the capabilities of dissident Irish groups, but at this stage we will not be ruling out any group who might see the secret intelligence service as a target."
Meanwhile, the Assistant Chief Constable of Sussex warned Brighton residents they may face greater disruption as a result of revised security measures for Labour's conference. The measures are likely to include increased numbers of road-blocks in Brighton and across Sussex. Mr Nigel Yeo said: "Our security operation for the conference has been many months in the planning but it has built-in flexibility to enable us to adjust our response according to the security threat."