The public prosecutor assumes that the 14-year-old schoolgirl who was killed in Munich was killed in her sleep.
"We assume a deliberate homicide and the homicidal trait of treachery," said the agency's spokeswoman, Juliane Grotz.
The girl was innocent and defenseless, the cause of death was a single stab in the chest.
The seventeen-year-old, who was arrested without resistance on Monday morning in connection with the homicide, had been friends with the schoolgirl for several months and had regularly stayed in the family's apartment, including on the night of the crime.
The girl and the teenager went to their room together on Saturday evening.
The next morning the mother of the fourteen-year-old found her daughter dead in bed.
The girl is said to have been killed between 4 a.m. and 6 a.m. in the early morning.
A resuscitation was unsuccessful.
The crime scene is in Denning, an eastern part of Munich.
The suspect was arrested by federal police officers in the Bavarian capital on Monday morning, as a police spokesman said.
A large number of police have been looking for the fugitive youth since Sunday.