The suspect, who is said to have hoarded war weapons in Aldenhoven west of Cologne, is a soldier of the German Armed Forces.
The Federal Ministry of Defense confirmed this on Thursday in Berlin.
The man was said to have worked in the operational command.
The 32-year-old has been in custody since Wednesday.
According to the ministry, the seized weapons and the confiscated ammunition did not come from the armed forces stocks.
However, "classified" documents were also found, which are now the subject of the investigation.
During a raid in Aldenhoven in the Düren district, investigators discovered an arsenal with war weapons.
The investigations by the Frankfurt public prosecutor's office are directed against a 32-year-old for violating the Foreign Trade Act and the War Weapons Control Act.
In a newly built district, the investigators came across firearms, grenades, hand grenades and mines.
Because unknown chemicals were discovered in the house, the emergency services consulted an expert from the Federal Office for Radiation Protection to evaluate the findings.
The process started after a find at Frankfurt Airport: Customs intercepted a package with silencers that the 32-year-old is said to have given up.
The authorities therefore initiated proceedings and obtained a search warrant.
A political background is currently not discernible, the public prosecutor announced on Wednesday.