A Swedish prosecutor says a 27-year-old German doctoral student is suspected of blackmailing the Czech government by sending an undisclosed toxic substance he had acquired over the Internet.

Prosecutor Henrik Soderman says the unnamed man also traded in toxic substances, like ricin, on the Internet. Soderman says ricin was not used in the blackmail scheme but declined to elaborate.

The man pleaded innocent to the charges but was put in pre-trial detention Saturday until April 29. He was arrested Wednesday at his home in Uppsala where he was a Ph.D. student at the city's university. The suspect's lawyer did not immediately respond to requests for comment.

Soderman said the investigation, which began in the fall of 2015, involved police in several countries, including the United States and the Czech Republic.