British anti-terrorist police warned the public to be on alert Tuesday after finding traces of ricin, a highly lethal poison that can kill even in tiny doses, during raids that led to the arrests of six Algerian men here.
Police did not say whether the suspects, rounded up in north and east London on Sunday, were linked to Al Qaeda or other terrorist groups. Police also did not say whether they thought the toxic substance was intended to be used in an attack.
But Scotland Yard officials expressed concern that the small quantities of ricin found may have been the residue of a production process. Investigators in protective suits continued their search of a one-room apartment where the poison traces were detected Monday night along with what authorities called suspicious equipment.
The discovery came after two months of interconnected raids by police in Britain and France targeting an Algerian-dominated network linked to Al Qaeda that is feared to be plotting attacks with bombs or weapons of mass destruction.
"We have previously said that London -- and, indeed, the rest of the U.K. -- continues to face a range of terrorist threats from a number of different groups," David Veness, the chief of the police anti-terrorist branch, and Pat Troop, the Health Department's deputy chief medical officer, said in a statement.
The Health Department warned doctors in Britain to be alert to symptoms of ricin exposure, including fever, stomach pains, diarrhea and vomiting. There is no antidote for the poison, which, when inhaled, causes respiratory failure. When ingested or injected, it can cause severe gastrointestinal symptoms leading to massive fluid loss and organ failure. Ricin can cause death within days.
In remarks to a meeting of British ambassadors Tuesday, Prime Minister Tony Blair said the case shows that the danger of weapons of mass destruction "is present and real, and with us now, and its potential is huge."
If the suspects turn out to be linked to Al Qaeda, this would be the first case in which police have discovered biological or chemical weapons in the possession of Islamic terrorists in Europe, according to European law enforcement officials.
Ricin is among the toxins that Al Qaeda terrorists trained with in Osama bin Laden's Afghan camps. Its manufacture was described in Al Qaeda manuals seized during past arrests in Europe, the officials said.
"This would be the first time something like this is found in Europe," a French law enforcement official said. "Certainly we know Al Qaeda people have talked about such weapons in the past."
Ricin is a protein toxin that can be extracted from the beans of the castor oil plant.
"The advantage of ricin for terrorists is the ease of manufacture," said Magnus Ranstorp of the Center for the Study of Terrorism and Political Violence at the University of St. Andrews in Scotland.
Ricin was the secret weapon in the 1978 assassination in London of Bulgarian exile Georgi Markov, who died four days after an assailant pricked him in the leg with a KGB-designed umbrella believed to contain just 450 micrograms of the poison. A microgram is one-millionth of a gram.
Nonetheless, ricin is difficult to disperse, making it a more potent tool for spreading terror than for inflicting widespread casualties, experts said.
"It is a weapon of mass disruption, not mass destruction," Ranstorp said.
Despite their great interest in using biological and chemical weapons, particularly cyanide gas, Al Qaeda and other terror groups haven't demonstrated sophisticated knowledge of such weapons, which generally are difficult to use, investigators said.
"It's not easy to work with this stuff and carry out an attack," said an Italian law enforcement official involved in an investigation in 2001 of Al Qaeda suspects in Milan who were wiretapped discussing techniques for using poison gas.
Even if terrorists haven't mastered spraying ricin via aerosol, they might attempt more primitive tactics such as spreading it on railings or turnstiles in public places, experts said.
Ricin also is among the biological weapons that Iraqi President Saddam Hussein's regime is believed to have produced, according to U.N. arms inspectors.
Britons have been confronted with grisly scenarios and government terror alerts since November, when police acting partly on a tip from French intelligence agents arrested three suspects, including an alleged leader of the Abu Doha network, an Algerian-dominated, London-based group implicated in plots to bomb Los Angeles International Airport and the cathedral in Strasbourg, France.
At the time, the British government denied tabloid newspaper reports that the suspected Al Qaeda operatives had scouted the London subway system, known as the Tube, for a planned attack using cyanide gas. But top French officials later said intelligence had indicated a danger of chemical attack in Britain.
Following up the British operation, French investigators in December captured associates of the London suspects among a dozen men, mostly Algerians, who have been accused of plotting attacks with explosives and chemical weapons in France.
During raids in the gritty suburbs of Paris, police found an anti-contamination suit used for handling toxins, bomb-making chemicals and documents containing formulas for cyanide that the suspects intended to acquire, the French law enforcement official said.
In Britain on Tuesday evening, authorities had little to say about the suspects, who range from teenagers to men in their 30s, or their suspected plans for the ricin.
A spokesman for Blair said there was "no specific intelligence about how this small quantity, this small amount of material, was to have been used. We have no specific intelligence on any direct threat, including to the Tube."
But the government urged vigilance and posted health advisories on official Web sites, noting that treatment should be "supportive" because there is no antidote.
Times staff writer Rotella reported from Paris and special correspondent Wallace from London.