A German man drove a van into a group of people sitting outside a restaurant in the old city centre of Muenster in western Germany on Saturday, killing two of them before shooting himself dead, police and state officials said.
Police tweeted Saturday afternoon that residents should "avoid the area near the Kiepenkerl pub" where a large-scale police operation is underway.
The suspect killed two people at the Grosser Kiepenkerl restaurant in the old town of the city in the northwestern region of North Rhine-Westphalia.
Der Spiegel reported that police were investigating a similar incident that occurred in the eastern German city of Cottbus on Friday evening, when a man drove his auto into a group of people, injuring two, before fleeing.
Police and prosecutors said because of the Muenster attack, they chose to mount raids on the suspects who are aged between 18 and 21. After a foreign intelligence service tipped off German authorities that he was planning to attack the half-marathon, police raided apartments and two vehicles in the Charlottenburg and Neukoelln districts of the city.
Muenster police spokesman Andreas Bode said the motive of the driver is still unknown and that authorities were looking into the possibility that other suspects may have fled the scene.
There was no known connection to radical Islam, he said, but investigators are still combing through the man's background.
Police said it was too early to speculate about the motive behind the crash and say they are still investigating.
"This was a awful and sad day for the people of Munster, all of Germany. and also the people of the Netherlands, who were sitting here and became victims", he said.
Munster is a popular tourist destination with 300,000 inhabitants, known for its medieval old town, which was rebuilt after massive destruction during the Second World War.
Police stands guard in a street near the spot. Dozens of ambulances were near the cordoned-off area and helicopters were flying overhead. "And the police arrived and everyone was sent out", he said.
"There were a lot of people screaming".
German Chancellor Angela Merkel said in a statement that she was "deeply shaken".
In a statement, Merkel said "everything conceivable is being done to investigate the crime and to support the victims and their relatives". "My thanks go to all the responders at the scene".
The attack is the latest in a string across Europe in which vehicles have been used to attack crowds of people in public places.
Grieshaber reported from Berlin.