Investigators in southern Germany said they have been unable to link a knife-wielding attacker who killed one man and wounded three others at a train station early Tuesday to any Islamist or extremist groups.

However, members of the State Criminal Investigation Bureau in Bavaria and prosecutors from Munich said they were still investigating a possible political motive, after witness reports that he had shouted "infidel, you must die."

Investigators said the 27-year-old German man lunged at four men at a train station in the town of Grafing at around 4:50 am (0250 GMT).

The first victim had boarded the first train of the day to the state capital, Munich, which is around 30 kilometres away, and was awaiting departure when he was attacked. The 56-year-old later died in hospital of his injuries.

The attacker went on to stab three other victims aged 43, 55 and 58, including two cyclists. One of the men remains in a critical condition.

Investigators dressed in white overalls cordoned of the crime scene, where bloody footprints could be seen on the railway platform.

The suspect, who was barefoot at the time, did not resist arrest and police were able to seize the weapon - a kitchen knife with a 10-centimetre-long blade - at the scene, investigators said.

Judging by an ID card and documents found near the crime scene, the suspect comes from the central German state of Hesse. It is unclear why he travelled to Grafing to carry out the attack.

Lothar Koehler, who heads Bavaria's State Criminal Investigation Bureau, said the man has been on unemployment benefits for the past two years. He arrived in Munich on Monday night, but slept rough around the city's main railway station because he could not afford a hotel, before travelling to Grafing early the next day.

Koehler described the man's attack as "more or less haphazard."

He said that the man had admitted to taking drugs in the days prior to the attack, adding that this may have been the reason for his confused testimony.

A Bavarian state minister said earlier Tuesday that the man, who has admitted to the crime, suffers from mental health and drug abuse problems, ruling out an Islamist motive behind the attack.

While authorities are continuing to investigate that possibility, Bavarian Interior Minister Joachim Herrmann said after a cabinet meeting in Munich that the man was clearly in a poor mental state.

German Interior Minister Thomas de Maiziere called the attack "despicable and cowardly," adding that "the motive for this crime has still not been conclusively explained."

Grafing Mayor Angelika Obermayr told German television that the stabbing attack in her "absolutely peaceful small town" had left residents "deeply" shaken.

"Something like this is completely new and shocks the people here really deeply," she said, adding that many had only encountered such horrors on television.

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