At least four people died on Saturday when a vehicle ploughed into busy cafe and restaurant terraces in the German city of Muenster before the driver shot himself dead.
Images on social media showed smashed and upturned tables and chairs strewn accross the pavement outside an eaterie in the centre of the picturesque medieval city.
At least 20 people were injured, six of them seriously, according to police, while media reports said the driver had been behind the wheel of a delivery van.
A police spokeswoman in the western city, Vanessa Arlt, told AFP that the driver of the vehicle "shot himself".
"The perpetrator drove into several cafe and restaurant terraces in a major square in the centre of Muenster," Arlt added. Police spokesman, Andreas Bode, said it was "too soon" to call the incident a deliberate attack.
According to the online edition of the Spiegel magazine, German authorities were "assuming" the incident was an attack, though there was no immediate official confirmation of a motive.
"Terrible news from Muenster," German government spokeswoman Ulrike Demmer said in a tweet, saying local police were providing regular updates.
"Our thoughts are with the victims and their loved ones." Images broadcast by German television showed police and firefighting vehicles clustered around a street in the centre of the city of 300,000 people.
Armed police were deployed and officers urged residents to avoid the city centre to allow investigators to get to work.
Germany had long warned of the threat of more violence ahead after several attacks claimed by the Islamic State jihadist group, the bloodiest of which was a truck rampage through a Berlin Christmas market in December 2016 that left 12 people dead.
The attacker, Tunisian asylum seeker Anis Amri, hijacked a truck and murdered its Polish driver before killing another 11 people and wounding dozens more by ploughing the heavy vehicle through the festive market in central Berlin.
He was shot dead by Italian police in Milan four days later while on the run. Germany has since been targeted again in attacks with radical Islamist motives.
In July 2017, a 26-year-old Palestinian asylum seeker wielding a knife stormed into a supermarket in the northern port city of Hamburg, killing one person and wounding six others before being detained by passers-by.