BERLIN (AP) — A call from the public was key to foiling an Islamic extremist's plan to use the toxin ricin to carry out a deadly attack in Germany, security officials said Wednesday.

The head of the country's domestic intelligence agency said a phone-in tip helped authorities confirm existing suspicions against a 29-year-old Tunisian man in the western city of Cologne. The suspect, named only as Sief Allah H. in line with German privacy laws, was arrested last week and remains in custody.

Hans-Georg Maassen, head of the BfV intelligence agency, said the alleged plot showed that the danger of an attack in Germany remains high.

According to federal prosecutors, the suspect bought at least 1,000 castor bean seeds online and used instructions posted on the web by the Islamic State group to make ricin. Investigators are still trying to determine whether the suspect, who is married to a German woman, was part of a concerted extremist plot or became radicalized on his own.

Tiny amounts of ricin can kill an adult if eaten, injected or inhaled.

German security agencies were first warned about the man by foreign intelligence agencies, but the information provided wasn't conclusive enough for authorities to act upon.

Holger Muench, the head of Germany's federal police agency BKA, told public broadcaster rbb-Inforadio that it wasn't clear whether the suspect had picked a specific target for his attack yet.

A Tunisian was behind the truck attack on a busy Christmas market in Berlin 18 months ago in which 12 people were killed. The perpetrator died in a shootout with Italian police days after the Dec. 19, 2016 attack, which was claimed by the Islamic State group.