The suspect “called the police station in (nearby town) Aalen at 12:48 pm to inform them he had shot several people,” police chief Reiner Möller told reporters in a press conference hours after the killings in Rot am See, near Baden-Württemberg state capital Stuttgart.
The young man stayed on the line, and when the first officers arrived minutes later at the hotel where the shooting took place, they immediately arrested him outside.
“They were then able to identify six dead people both inside and behind the building,” Möller said, including three men aged 36, 65 and 69 and three women aged 36, 56 and 62.
Images from the scene showed large numbers of emergency vehicles and heavily armed officers sealing off the area with red and white police tape.
Meanwhile forensics teams dressed in white coveralls moved in to secure evidence.
Two survivors of the attack are receiving medical treatment, with one of them “in danger for his life,” police chief Moeller said.
The shooter had also threatened two children aged 12 and 14, leaving them shaken but unharmed.
Investigators have so far been unable to discover anything about the motive of the suspect, saying he would be questioned only when his lawyer arrived at the police station.
So far the police have been able to confirm only that two of the dead were the suspect's father and mother.
“We are still clarifying the other relationships” between the group, Möller said.
The perpetrator himself, a German citizen, lived in the hotel along with some of the victims, near the station in the town of 5,200 people.
German media had earlier reported that the group had met in the hotel for a family gathering, but police have so far been unable to confirm this.
Investigators say the crime was committed with a semi-automatic handgun, for which the suspect held a license for sport shooting.
The weapon was found inside the building after officers arrested the 26-year-old man.
Police at the scene. Photo: DPA
While owning firearms is not illegal in Germany, most guns can be acquired only with a licence and they are closely monitored, making mass shootings comparatively rare.
In October last year, a far-right attacker shot two people dead in the eastern city Halle, wounding several more after failing to break into a packed synagogue armed with home-made weapons.
In July 2016, a teenager used a pistol bought illegally online to kill nine people in a Munich shopping centre, before turning the weapon on himself.
Germany has also been the target of a number of jihadist attacks in recent years, although most of the perpetrators did not use guns.
The most deadly took place in December 2016, when Tunisian Anis Amri drove an articulated truck into a Berlin Christmas market, killing 12 people.
By Tom Barfield