The son of former German President Richard von Weizsäcker has been stabbed to death at a Berlin hospital.
Dr Fritz von Weizsäcker, 59, had just given a lecture on liver diseases. His father served as president during the reunification of Germany in 1990.
A 57-year-old man was taken into police custody and then sent to a psychiatric unit.
An off-duty policeman was seriously injured when he stepped in to overpower the attacker.
The 33-year-old officer is now out of danger after surgery, say police.
Prosecutors say the suspect has an "acute mental illness" and that his motive was a "probably delusional, general dislike" of the von Weizsäcker family.
They say he bought a knife in the south-western state of Rhineland-Palatinate before travelling to Berlin by train to carry out the attack. His apartment in the town of Andernach, on the bank of the Rhine, was searched on Wednesday, DPA news agency reports.
The suspect has been charged with both murder and attempted murder.
German shock at 'terrible blow'
The attack occurred at the Schlosspark hospital in western Berlin neighbourhood of Charlottenburg, where Dr Weizsäcker worked. Despite efforts to save him, he died at the scene.
The policeman, who happened to be in the audience, was one of some 20 people who intervened. He was taken to another hospital to be treated for his injuries.
Richard von Weizsäcker served as president of West Germany from 1984 to 1990. He was in post during the reunification of West and East Germany, continuing to serve as president of a united Germany until 1994. He died in 2015.
Christian Lindner, head of the liberal Free Democratic Party, said on Twitter that he was "stunned" by the death of his friend, adding that "once again we ask ourselves what sort of world we are living in".
Chancellor Angela Merkel spoke of a "terrible blow to the Weizsäcker family" and expressed her condolences.
Environment scientist Ernst Ulrich von Weizsäcker praised his "wonderful" cousin, while the victim's sister, Beatrice, posted an image of a cross without a comment.
Although not a public figure, Fritz von Weizsäcker is believed to have been targeted because of his late father. Several German politicians have been attacked in recent years:
- June 2019: Pro-migrant CDU party official Walter Lübcke shot dead, allegedly a victim of a far-right attack
- November 2017: Pro-migrant mayor Andreas Hollstein survives being stabbed in the neck in a kebab shop
- October 2015: Henriette Reker survives being stabbed in the neck shortly before she is elected mayor of Cologne
- 1990: Interior Minister Wolfgang Schäuble is shot and paralysed during a campaign meeting
- 1990: Candidate for chancellor Oskar Lafontaine survives being stabbed in the neck by a woman with a history of mental illness