The 36-year-old man was previously already probed over suspected ties to the Islamic State group, said public broadcaster Hessischer Rundfunk.
Prosecutors have broadened their investigation after finding bomb-making instructions in his possession.
The suspect, who was studying for a doctorate at western Germany's Darmstadt Technical University, was detained last year over a video broadcast online in which he allegedly voiced his support for the IS group.
Investigators had confiscated his mobile phone and other electronic devices, and subsequently found the bomb-building instructions.
Germany is on high alert for jihadist attacks following a string of assaults last year, the deadliest of which was an IS-claimed December 19 truck rampage through a Berlin Christmas market that killed 12 people.