Police searched several apartments in west and southwest Germany on Friday in connection with an incident that had prompted security to be stepped up at Stuttgart Airport the previous day.
“Nobody has been taken into custody yet,” a spokesman for the Stuttgart prosecutor’s office said.
The news weekly Der Spiegel earlier said on its website that prosecutors were investigating four people suspected of having planned a serious act of violence.
Two of the suspects – a man and his son – had ties to Islamists in the western German city of Aachen, it said.
Police had said on Thursday they had massively boosted security at Stuttgart Airport in response to a threat they did not specify.
German media, citing security sources, said suspected Islamist militants had been spotted at the airport in what could have been reconnaissance operations.