In the case of the 14-year-old girl from Gottenheim near Freiburg, who was found dead in Teufelsee near Echzell, a 29-year-old man was arrested on strong suspicion.
On Monday, the public prosecutor's office and the police in Freiburg reported on their findings.
Investigators tracked 250 leads from the public and ten million digital leads.
The latter led to the suspect, a convicted sex criminal.
The 29-year-old German is now in custody in Freiburg.
The charges are child confiscation, sexual assault and murder with intent to cover up.
During the interrogation, the accused initially denied the allegations, since then he has remained silent.
14-year-old Ayleen disappeared on Thursday July 21.
A day later she was brought to Hesse by a man in a car.
On July 29, Ayleen's body was found.
Due to the long time spent in warm water, the pathologists were unable to determine the cause of death despite a seven-hour autopsy.
It cannot be ruled out that the girl drowned, said senior public prosecutor Dieter Inhofer.
The suspect had attacked an eleven-year-old girl from behind in 2007 at the age of 14 and had been convicted of attempted rape by the Wetzlar district court.
He was put into a correctional facility and was released from psychiatric treatment in 2017.
The subsequent management supervision for the surveillance of sex offenders at risk of recidivism in Hesse ended at the beginning of 2022.
Ayleen and the twenty-nine-year-old knew each other from chats on the Internet.
The investigators assume that the contact lasted several weeks via social networks and the online game Fortnite.
There are indications that Ayleen left the place of disappearance alive, the prosecutor said.
It is not known whether she got into the car voluntarily.
Cell phone data suggested that "something may have happened" on the night of July 21-22.
The digital traces led to Hesse.
During a search of an apartment near Wetzlar, the police found Ayleen's personal belongings, reported the Hessian LKA President Andreas Röhrig.
The suspect was in Frankfurt, where he was arrested.
No other evidence was found during a search of his car.
Cell phone recordings showed that the man had stayed at Teufelsee for a long time.
Emergency services were then dispatched there.
A body floating in the water was spotted from the police helicopter.
The small lake in a nature reserve is surrounded by scrub and cannot be reached directly by car.
The senior public prosecutor emphasized that the investigation is still in its infancy.
Even if there is an urgent suspicion, the presumption of innocence applies to the man.
Therefore, no further details about him or his individual background would be released.