“We assume there is a connection,” said state prosecutors in Heidelberg in a statement after a string of mysterious attacks on a supermarket chain, a drinks company and a baby food producer earlier this week.
Further attacks were “not likely” but “could not be ruled out entirely”, they added.
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Prosecutors announced Thursday the launch of a 100-person special commission to investigate the attacks.
They also confirmed that an explosive had been identified and disarmed at a parcel distribution centre at Munich airport on Wednesday night.
The parcel, which was addressed to the Bavaria-based baby food company Hipp, was the third such attack in a matter of days.
On Wednesday, three people were taken to hospital when a letter bomb exploded at the German headquarters of discount supermarket Lidl.
Around 100 people were evacuated from the administrative building at the company's head offices in Neckarsulm, western Germany.
A similar explosion was also reported in nearby Eppelheim at the Wild drinks company, whose products include Capri-Sun brand.
According to national news agency DPA, Germany's food federation had also warned its members to remain vigilant when receiving post.