(BLOOMBERG) - An attack in the northern city of Hamburg thrust Chancellor Angela Merkel's refugee policy into the spotlight less than two months before Germany goes to the polls.

Police were aware that the man suspected of a stabbing spree on Friday (July 28) had shown of signs Islamic radicalisation, Hamburg's interior minister, Andy Grote, said in a press conference on Saturday.

The attack in a supermarket in the Barmbek district left one man dead and six injured before bystanders overpowered the attacker.

The 26-year-old suspect is a Palestinian citizen whose request for refugee status had been declined. He had been ordered out of the country but lacked the necessary documents to travel, Grote said, adding that a few hours before the attack the man had inquired whether his documents were ready. The suspect, who had shown signs of mental instability, is now in police custody.

"I grieve for the victim of the cruel attack in Hamburg, and I am deeply sympathetic to his relatives. I wish the wounded to recover from their physical and mental wounds," Merkel said in a statement on Saturday afternoon. "The act of violence must and will be clarified."

While authorities said they are still unsure of the motives of the attacker, the episode could damage Merkel's September re-election bid, which is based in part on ensuring public safety. The anti-immigration Alternative for Germany party was quick to react.

"After Barmbek, Merkel's Christian Democratic Union party has lost any credibility on internal security and should stop campaigning on the issue," AfD's deputy speaker, Beatrix Von Storch, said on Twitter.

MERKEL'S SUPPORT

Before the attack, support for Merkel, who is seeking a fourth term in the Sept 24 federal election, was at the highest level since the peak of the refugee crisis nearly two years ago. An Infratest dimap poll for ARD television showed Friday that her CDU-led bloc had 40 per cent support, widening the lead over her Social Democrat challengers to 17 percentage points. AfD polled third at 9 per cent, after rising to as much as 15 per cent when immigrant arrivals to Germany dominated headlines.

Merkel's main challenger in the election, Social Democrat leader Martin Schulz, expressed his "shock" at the attack and offered his "deepest sympathy" for the victims and their families in a tweet on Saturday morning. Schulz has also made immigration a theme of his campaign, travelling to Italy this past week to witness first-hand the situation in a country where many asylum seekers first arrive in Europe.

"We must take into account that the jihadist ideology is used as reason or justification for acts that are perhaps committed for quite different motives,"

German Interior Minister Thomas de Maiziere, a member of the CDU party, said in a statement. "The real motives can then also lie in the personality of the attacker."

In the immediate aftermath of the attack, Hamburg's Social Democrat mayor called for swifter deportations for failed refugees.

"What makes me especially angry is that the offender is apparently someone who claimed protection by us here in Germany and then directed his hate against us," Hamburg Mayor Olaf Scholz said in a Facebook post.

"This shows all the more urgently that these legal and practical obstacles must be cleared away during the deportation process. These perpetrators are anxious to poison our society. They will fail."

DECEMBER INCIDENT

Hamburg's incident is reminiscent of an attack in December at a Berlin Christmas market where the perpetrator, from Tunisia, killed 12 people. In that case, the man's asylum application had been rejected but he hadn't been deported because he lacked a passport.

The Berlin incident was the last - and most deadly - in a series of assaults involving refugees that shook Germany last year in the wake of an influx of 1 million asylum seekers into the country in 2015.

The incidents soured the German public and led Merkel last summer to vow tougher action on deporting people who shouldn't be in the country after her party suffered setbacks in two state elections. With no deadly attacks involving asylum seekers this year until Friday's Hamburg knifing, Merkel's Christian Democratic Union has rebounded in the polls and won three straight state votes.

A witness to Friday's assault told Deutsche Presse Agentur that the man shouted as he held up the knife, "Allahu Akbar," Arabic for "God is greatest." Police searched a refugee shelter where the attacker lived, the news agency reported on Saturday.