Police said they were searching for more ricin toxin and possible conspirators in a plan to use it after detaining a seventh suspect in Britain.
London's Metropolitan Police said Wednesday that more arrests were still possible in connection with the discovery of ricin in a London apartment. The seventh suspect was arrested Tuesday.
A police spokesman said detectives were worried "that there is a quantity (of ricin) out of our control which we are still looking for."
Police believe the suspects had intended to use the poison to kill a small number of people in hope of terrifying Britons.
"We don't think that the intention was to do a mass attack, but we thought it would be more for smaller targets, one or two people, just to cause fear and panic," he said, speaking on condition of anonymity.
The British Broadcasting Corp. and ITV both reported Wednesday night that police were stepping up security for British politicians, public buildings and transportation centers involving trains, subways and ferries.
The police spokesman dismissed as speculation reports that the men may have intended to assassinate a high-profile politician, possibly Prime Minister Tony Blair.
Police did not identify the seventh suspect, saying only that he was 33 years old and was being held at a London police station. They said the first six suspects were in their late teens, 20s and 30s, and were of North African origin. They would not specify what country or countries they came from, declining to confirm media reports that the men are Algerian.
Police said nothing about the background of the seventh man. None of the seven have been charged with a crime. Investigators were questioning the suspects and continuing to search the north London apartment where they found the ricin.
Six of the men were arrested Sunday in north and east London. Police said Tuesday that material seized from an apartment in the Wood Green neighborhood tested positive for traces of ricin, which can kill within days. There is no antidote or treatment.
A spokesman for the local government council in the Islington section of north London said it had paid to house two of the suspects in the apartment where the poison was found. He said they were seeking political asylum and were between 16 and 18 years old.
Blair's spokesman said the ricin discovery clearly indicated the danger international terrorism poses to Britain.
"The British public has long experience of the threat of terrorism," he said, briefing reporters on condition of anonymity. "They understand the need to be vigilant, but they also understand the need not to do the terrorists' job for them" by panicking.
The Department of Health warned doctors in Britain to be on the lookout for symptoms of exposure the poison, including fever, stomach ache, diarrhea and vomiting. Ricin is twice as potent as cobra venom.
Ricin (pronounced RICE-in) is derived from the castor bean plant, which is grown around the world, and is relatively easy to produce. It has been linked in the past to Usama bin Laden's Al Qaeda terror network and Saddam Hussein's regime in Iraq.
In Washington, U.S. officials said investigators had found no links between Al Qaeda and the London arrests, but were looking into the possibility.
U.S. officials said in August that the Islamic extremist group Ansar al-Islam had tested ricin along with other chemical and biological agents in northern Iraq, territory controlled by Kurds, not Saddam. The group is allegedly linked to Al Qaeda.