Federal prosecutors have found more than 3,000 castor bean seeds in the suspect's Cologne apartment — many more than initially suspected. They say the Tunisian was planning to make a biological weapon using ricin.
The 29-year-old man, identified only as Sief Allah H. in accordance with German privacy laws, had manufactured ricin, a poison found in castor beans, for the suspected attack, the president of the Federal Criminal Police Office (BKA), Holger Münch, told German broadcaster RBB-Inforadio.
"There were very concrete preparations for such an act using what you might call a biological bomb," Münch said, describing it as an "unprecedented" threat.
According to federal prosecutors, about 3,150 castor bean seeds — more than three times the number initially suspected — and 84.3 milligrams of ricin were found in the suspect's apartment.
Ricin is 6,000 times more potent than cyanide and is lethal in minute doses if swallowed, inhaled or injected. It has no known antidote.
Münch said that objects that could be used to make a bomb were also found in the searches.
The target of the suspected attack was not clear, he added.
Federal prosecutors said the man had been in contact with "persons from the radical Islamist spectrum," and that they were still probing the content of the communications.
"There are as yet no leads indicating that the accused was a member of a terrorist group," the federal prosecutors' office in in the southwestern city of Karlsruhe said.
But they said the man, who is married to a German woman, had twice tried to travel to Syria last year.
Prosecutors say the suspect bought the seeds online, and used instructions posted online by the "Islamic State" (IS) militant group to make ricin.
Germany's domestic intelligence agency chief Hans-Georg Maassen said it was very likely that a terrorist attack had been thwarted.
Maassen said a phone-in tip helped authorities confirm their suspicions about the Tunisian and foil the extremist's plan.
German security agencies first were warned about the man by foreign intelligence agencies, but the information provided wasn't conclusive enough to act upon, authorities said.
Maassen, who heads the Office for the Protection of the Constitution (BfV), added that the alleged plot showed that the danger of an attack in Germany remains high.
Europol, Europe's police agency, warned on Wednesday that the risk of terrorist attacks by Islamist militants "remains acute."
"As IS gets weaker, it has been urging its followers to carry out lone actor type attacks in their home countries, rather than guiding them to travel to the so-called caliphate," Europol said.
Armed jihadis plotted or carried out more attacks in Europe last year than in 2016, with a total of 33 attacks either planned or executed, the law enforcement agency said in a report.