(CBS) 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. Six other men, of North African origin, arrested on Sunday in north and east London were being questioned, London's Metropolitan Police said.

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.

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.

Home Office Minister Beverley Hughes is appealing for calm, and most Londoners are going about their business, reports CBS News Correspondent Steve Holt, but people living near the north London home where the ricin was found are understandably upset.

"It's been very horrible. It's right under our nose, and nobody knew anything," said one neighbor.

"It worries us because we only live two, three meters from the place," said another.

"It's understandable that people are concerned," Hughes told the British Broadcasting Corp. "But we need to keep it in perspective. People need to be alert, but not alarmed and panicked."

The Department of Health warned doctors around Britain to look for symptoms of exposure to ricin, which has no antidote. Symptoms include fever, stomach ache, diarrhea and vomiting.

"We have alerted general practitioners, other doctors, specialists, the public health people, the poisons service and so on to be alert to any symptoms so that they can advise patients should they have any concerns, said Deputy Chief Medical Officer Pat Troop.

"I should point out that most people who have those symptoms at the moment, they most probably have got flu, so I don't want people to be alarmed if they have a temperature that they should be concerned," he added. "But we just have to be alert to the fact that there maybe some of this material available."

Anti-terrorist police said Tuesday they had found "material and items of equipment" in an apartment in north London's Wood Green district, and that a "small amount of the material" tested positive for ricin. One of the men was arrested in the apartment, police said.

Terrorism expert Dr John Gearson called the find significant.

"It shows al Qaeda or some sort of terrorist organization is planning to try and have an attack of some sort, possibly to achieve mass effect, rather than mass casualties, but they are certainly based in Britain and planning to do something," he said.

It's not Britain's first brush with Ricin, reports Holt. In 1978, Bulgarian exile Georgi Markhov was killed on a London street by an assassin who fired a ricin pellet from an umbrella modified into a gun.

Dr. Rufus Crompton, the pathologist on that case, remembers it was extraordinarily hard to diagnose, because Ricin produces symptoms mimicking natural diseases.

"If we hadn't found that pellet, Markhov would have been signed up by the coroner as blood poisoning," Crompton said.

The Daily Mirror newspaper showed a skull and crossbones against a map of Britain on its Wednesday front page under the headline "It's here." The Sun, Britain's biggest circulation daily, said the discovery revealed a "factory of death."

The public health director for London, Sue Atkinson, urged people not to be alarmed. "What has been found is a very small amount of this and it's quite difficult to perhaps use it for mass destruction," she told the BBC.

But Prime Minister Tony Blair said Tuesday that the find highlighted the threat posed by weapons of mass destruction.

"As the arrests ... show, this danger is present and real, and with us now, and its potential is huge," he said.

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 Qaeda terror network and Saddam Hussein's regime in Iraq.

In Washington, U.S. officials said Tuesday that no al Qaeda links had yet been established to the London arrests, but that it was a matter being investigated.

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.

United Nations 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 Qaeda biological weapons sites in Afghanistan.