Police said on Friday they were conducting an all-out search for the victim's 23-year-old brother, whom they "strongly suspected" was responsible for the attack in a park in a central area of Hamburg on Thursday night.
A police spokeswoman said the motive was unclear in response to German media speculation that the girl's death from multiple stab wounds was the result of an honor killing.
"We are investigating in all directions," she said.
Locals heard the girl's screams and called for help, but paramedics arriving at the scene were unable to save her, police said.
Concern about honor crimes -- motivated by a desire to defend traditional ideas of family reputation, often when the victim has become involved in what is seen as an unsuitable relationship or way of life -- is growing in Germany, which has the second-largest Muslim population in western Europe.
In a high profile case in 2006, a man of Turkish descent was jailed for shooting dead his sister at a Berlin bus stop after she left her husband to live alone.
Source: Reuters (English)