German public prosecutors are investigating a 29-year-old man on suspicion of preparing a "serious act of violent subversion" after police found an "unknown substance" during a raid of his apartment late on Tuesday.
"We need to wait for the forensic results," a spokesman said, adding that they had not ruled out terrorism.
Daily newspaper Express and local daily Kölner Stadt-Anzeiger reported that police had found ricin, a substance that can be highly toxic in small amounts. Authorities have not confirmed the reports.
Police have said they arrested the suspect, a Tunisian citizen, along with his wife in the northern neighborhood of Chorweiler in Cologne, a city in the state of North-Rhine Westphalia. German news agency dpa reported that the wife is not suspected of any involvement.
Elite SEK police commandos and specialized chemical units from the police and fire departments were at the scene of the 18-story apartment block Chorweiler late on Tuesday. A mobile laboratory was set up in the entrance area of the apartment to analyze the substance.
amp, cw/sms (dpa, AFP)