In the case of the two children who died violently from downtown Hanau, the suspect, the father of the siblings, was extradited to Germany.
The 47-year-old man was arrested in a Paris suburb eight weeks ago after the crime and was arrested in France.
Officials from the Hessian State Criminal Police Office and detectives from Hanau picked up the suspect in France on Thursday and took him to the district court in Hanau, according to the Hanau public prosecutor's office.
Jan Schiefenhoevel
Editor in the Rhein-Main-Zeitung.
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There he was brought before an investigating judge who ordered him to be held in custody.
Now the man is in prison in Frankfurt, according to the public prosecutor.
According to a spokeswoman, the suspect did not comment on the allegations before the judge.
To the French police he had denied an act of violence against the children.
In May, an eleven-year-old boy was found seriously injured in front of a residential building in downtown Hanau.
The child died a little later in a hospital from internal injuries, which were probably caused by a fall from a balcony.
His seven-year-old sister was found dead on a balcony on the new floor of the apartment building.
The girl's body had injuries to her neck.
The siblings lived with their mother in the high-rise building.
The family moved to Hanau earlier this year.
Because of family difficulties even before the act of violence, the Hanau youth welfare office had become active.