State prosecutor Klaus-Dieter Schultz said the offices as well as the control room at the Krümmel plant were searched after Vattenfall refused to release the name of the operator who was on duty when the fire broke out on June 28th.
The incident has caused outrage in Germany, with the environmentalist Green party calling for Vattenfall’s licence to operate nuclear power plants in Germany to be revoked.
The fire broke out just hours after the nuclear reactor at nearby Brunsbüttel, which is operated by the same company, had to be shut down temporarily because its capacity was overloaded.
Both plants are situated in the northern German state of Schleswig-Holstein.
The regional social affairs ministry has accused Vattenfall of failing to communicate the full extent of the problems at the two plants.
The company initially said the fire at the Krümmel plant had been isolated from the atomic reactor, but according to the ministry the flames had reached the building housing the reactor.
It also says Vattenfall waited five days to report technical problems that occurred when workers tried to restart the Brunsbüttel plant.
The Swedish company has denied any cover-up but its role in running Germany’s nuclear power plants has become contentious in a country deeply divided about whether it should be using nuclear power at all.
German Chancellor Angela Merkel entered the fray this week with a demand that Vattenfall explain exactly what happened at the Krümmel plant.
Officials in Schleswig-Holstein are due to question the shift leader who was on duty when the blaze began on Monday.