A Syrian man was arrested on Tuesday on suspicion of plotting a “radical Islamist” attack in Germany.
The 28-year-old man is suspected of buying materials on eBay to make a home-made explosive belt.
He was arrested by members of the special forces during raids in Hamburg.
Police are also investigating his 24-year-old brother over suspected links to the plot.
The two brothers planned to "carry out an attack against civilian targets", a police statement said.
It said the exact target was not clear.
The 28-year-old man is accused of breaking terrorist financing laws by gathering assets to plan an attack.
He had allegedly been buying materials on eBay and from other providers "for several weeks".
Chemical substances were found during Tuesday's raids, which involved 250 officers, police said.
People who had contact with the two men also had their homes searched.