German police are searching flats in a Cologne tower block where a Tunisian man is suspected of having kept ingredients for highly toxic ricin.
The 29-year-old man, named in German media only as Sief Allah H, is being questioned by police.
Police stormed his flat on Tuesday and found a chemical which turned out to be ricin. He is suspected of planning a biological terror attack.
Ricin is a poison found naturally in castor beans.
German security sources quoted by RP news, which is based in the Cologne region, said the quantity of ricin found was enough for up to 1,000 toxic doses.
Police have sealed off the apartment block in Cologne-Chorweiler district. They are searching two flats rented by the suspect, as well as six other empty flats and some public areas in the building.
The authorities say there is no immediate danger to the other residents.
Experts from the Robert Koch Institute – a prestigious scientific research centre – are with police at the scene.
Germany’s top constitutional protection official, Hans-Georg Maassen, said it was “very probable that a terror attack was foiled here”.
Ricin can be made from waste left over from processing castor beans, the US Centers for Disease Control (CDC) says. It can take the form of powder, a mist, or a pellet, or it can be dissolved in water or weak acid.
Ricin made headlines internationally when it was found to have killed a Bulgarian dissident, Georgi Markov, in an infamous umbrella stabbing in London during the Cold War.
The exiled writer and BBC journalist, an opponent of Bulgaria’s then-Communist government, was stabbed in the thigh in 1978. Later a tiny hollow pellet was found in his body.
German media report that the suspect in Cologne was investigated after he had bought 1,000 castor seeds and an electric coffee grinder on the internet.