Muddy bootprints along the corridors marked the tell-tale signs of a massive police raid at an asylum seeker shelter in Hamburg, where the suspect who killed one and wounded six in a knife attack lived.

Shell shocked women and children peeked out of rooms lining the first floor of block 4D, as neighbours of the 26-year-old suspect recounted his sudden transformation from a helpful man to a religious fanatic.

‘He was always helping other asylum seekers with their paperwork,’ his neighbour, who gave his name only as Mohamed, told AFP.

‘He was a really good guy, never aggressive,’ said the neighbour.

But in recent weeks, he ‘had a crisis, he bought Islamist clothes and read the Koran very loudly in his room,’ added the 31-year-old Syrian refugee.

‘And three weeks after Ramadan, he had another crisis. He started to drink heavily and smoke joints... he was sad that his mother was ill and that his asylum request was rejected,’ recounted Mohamed.

A day after the suspect stormed into a supermarket on a busy high street in the north-east of the port city, and launched an attack using a knife he took from the shop's shelves, a picture is starting to emerge of his background.

Named by Spiegel magazine as Ahmad A., he had arrived in Germany in March 2015, after having already spent some time in Norway, Sweden and Spain.

Having lived for several years in Scandinavia, he spoke fluent Norwegian, said Hamburg prosecutors.

He filed for asylum in May 2015, and according to his birth certificate, was born in the United Arab Emirates.

He was eventually sent to the Hamburg asylum shelter, a container village with some 600 residents in the leafy northern suburb of Hamburg.

The container village totalled seven blocks, with two single beds on either side of each 15 square metre room.

In late 2016, his asylum application was rejected although he could not be deported immediately because he lacked identity papers.

Officials said he appeared to accept that he would have to leave Germany, and even helped in the process to obtain documents to facilitate his departure.

On the day of the attack, he had even gone to the authorities to ask if the identify papers had arrived. Police chief Ralf Meyer said the suspect was ‘almost exemplary’ in this aspect.

But police had also noted that the man had been radicalised and deemed him an ‘Islamist’.

He was also psychologically unstable, said authorities.

Those living close to him at the shelter also noted that his behaviour had become more erratic in recent weeks.

He often took drugs, and then he shouted Islamist slogans, one of the fellow residents at the shelter told Bild daily.

For Mohamed, the entire saga was a tragedy.

‘I find this whole story very sad. He is 26 years old and... he has not achieved anything. And now we don't know why he did this,’ he said.