German customs agents have raided the offices of 41 companies suspected of links to Iran's nuclear programme.
Germany has been among EU states negotiating with Iran
Prosecutors said specialised equipment worth several million euros that could have been used for a nuclear programme had been shipped to Iran via Russia.
The suspicious exports were traced to a Berlin firm no longer in business.
Iran could face United Nations action over its nuclear programme, which it insists is peaceful and not aimed at producing nuclear weapons.
Prosecutors said seven men, mostly of Russian origin, were suspected or organising the network.
The men have not been arrested and their whereabouts have not been confirmed.
Benedikt Welfens, a spokesman for the German prosecutors' office, told the ARD TV network that the Berlin-based company acted as a cover for a network involved in exports to Iran.
"Its main business is the supply of Iran's nuclear programme," Mr Welfens said.
"We found out that the employees of this company purposefully contacted companies in Germany in order to purchase very specific components for Iran."
He suggested that the goods were worth up to three million euros ($3.6m; £2m).
"It's not that much, but it may rise. We have to look at the information and data that we've found," he said.
Germany has strict rules against selling sensitive technologies to a number of countries and police are currently working on two other cases.
An investigation is underway against four men suspected of trying to obtain missile technology for a foreign intelligence service and a German engineer is on trial for allegedly helping Libya's now defunct nuclear programme.