"We have no clear information as to the motive or the number of wounded".
But a year ago an evaluator who interviewed the Palestinian - who spoke fluent English, Swedish and Norwegian - said that while he had mental health problems, he did not pose an imminent danger, Mr. Voss said.
Around 3:10 pm (1310 GMT), a man carrying a large knife entered the supermarket belonging to the Edeka chain on Fuhlsbuettler Strasse, a lively shopping street in the Barmbek district of northern port city Hamburg.
German media are dubbing the men who went after the attacker with chairs - reportedly mostly Muslims of Arab and Turkish descent - as the "Heroes of Hamburg".
Hamburg Mayor Olaf Scholz said the attack was motivated by "hate" and said the suspect's deportation had been delayed due to lack of necessary papers. "They said he started talking a lot about the Quran, stopped drinking alcohol and questioning many things".
Police said passersby tackled the man after he fled the scene, injuring him slightly, before plain clothes police officers could take him into custody.
Homicide investigators are leading the probe, said police, adding that they were pursuing "all leads" for a motive.
In 2016, ISIS also claimed a suicide bomb attack in the southern city of Ansbach which wounded 15 victims, and a 17-year-old Afghan migrant's axe rampage on a train in Bavaria injured five, before the Afghan was shot and killed by police.
The 26-year-old failed asylum seeker is now being held on suspicion of one count of murder as well as five counts of attempted murder, spokeswoman Nana Frombach said.
Prosecutors said the attacker pulled a 20cm knife from a shelf at the supermarket and stabbed three people inside and four outside. The Berlin incident was the last - and most deadly - in a series of assaults involving refugees that shook Germany previous year in the wake of an influx of 1 million asylum seekers into the country in 2015.
In the early hours of Saturday, German police searched the Hamburg refugee shelter where the suspect had lived.
Police then arrested the attacker.