According to the country's prosecutor's office, the 17-year-old is suspected of having offered his services as a contract killer on social media. He was then allegedly recruited by an unknown person, reports German broadcaster WDR.
The 17-year-old was assigned to kill an unidentified person with a firearm.
He was arrested after a tip from Swedish police. Police in Germany raided a room the 17-year-old rented in Cologne and seized a firearm and ammunition.
The case may be linked to an attempted murder of a member of the Hells Angels motorcycle club this weekend, German media reports.
