LONDON -- Partners of seven men arrested in connection with the discovery of the deadly toxin ricin in a London apartment may still be at large with more poison, police said Wednesday.
London's Metropolitan Police announced they had apprehended a seventh suspect in the case Tuesday and added that their inquiry was still active, with more arrests possible.
A 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 -- one of the world's most potent toxins -- 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 spokesman dismissed as speculation media reports that the men may have intended to assassinate a high-profile politician, possibly Prime Minister Tony Blair.
But 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 Metropolitan Police said it never discusses the details of its security operations, especially regarding possible terrorist threats.
Health authorities urged doctors around the country to be alert for signs that any Britons might have been poisoned.
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, but would not specify what country or countries they come 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 a flat 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 council in the Islington section of north London said it had paid to house two of the suspects in the flat 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. "Its character is strong in testing times, and I think people understand that there is a threat. They understand the need to be vigilant, but they also understand the need not to do the terrorists' job for them" by panicking.
Ricin -- twice as potent as cobra venom -- was used in the murder of Bulgarian defector Georgi Markov in London in 1978. Police said the toxin was in a pinhead-sized pellet injected into Markov's thigh, but couldn't confirm the widely reported theory that he was jabbed by a rigged umbrella.
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 (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 Osama bin Laden's al-Qaida terror network and Saddam Hussein's regime in Iraq, and is also favored by white supremacist groups in the United States.
British media reacted to news of the discovery with alarm.
The Daily Mirror tabloid showed a skull and crossbones against a map of Britain on its front page under the headline "It's here," and the Daily Mail printed a photograph of a man in a gas mask next to the words "Poison Gang on the Loose."
The government urged calm.
"It's understandable that people are concerned," Home Office Minister Beverley Hughes told the British Broadcasting Corp. "But we need to keep it in perspective. People need to be alert, but not alarmed and panicked."
London's public health director, Sue Atkinson, told the BBC it would be very difficult to use ricin for a large-scale attack.
In Washington, U.S. officials said investigators had found no links between al-Qaida 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-Qaida.
U.N. weapons inspectors who left Iraq in 1998 listed ricin among the poisons they believed Saddam produced. U.S. troops also found traces of the substance at suspected al-Qaida biological weapons sites in Afghanistan.