The 26-year-old migrant who killed one person and injured six others in a knife attack in a Hamburg supermarket on Friday was known to German security forces as an Islamist, the city-state's interior minister said.

Andy Grote told a news conference on Saturday that an initial investigation found that the man, an asylum seeker who could not be deported because he had no identification papers, is also believed to have psychological problems.

Hamburg police chief Ralf Martin Meyer said that while initial findings showed the attacker had acted alone it could not be completely ruled out that he had accomplices.