Hamburg stabbing suspect a known radical

German officials on Saturday said the 26-year old man, identified as Ahmad A, was known to security services.

Believed to be a Palestinian asylum seeker, the man had no identity papers other than a birth certificate showing he was born in the United Arab Emirates.

"At the moment, we can say about the offender's motive that there is a reference to religious motivations on the one hand – Islamic motives - but on the other hand, there are indications of psychological instability," said Hamburg state interior minister Andy Grote.

At a supermarket in Hamburg's Barmbek district on Friday, the attacker pulled a 20-centimetre knife from a shelf and stabbed three people inside and four outside the premises.

But he was quickly overwhelmed by passers-by who threw chairs and other objects at him, giving time for the police to arrest him.

Although the man's motive remained unclear, authorities believe he acted alone.

READ: One dead in knife attack in Hamburg supermarket, motive unclear

"We have to determine whether the offender was integrated in Hamburg's Islamic extremist scene or not. For the moment, we don't have indications of (him belonging to) a network," Grote said.