Named by Bild newspaper as Alyssa B., a passer-by found the teenager in a forest on the south-east edge of Berlin at around 3pm on Monday afternoon with a large knife stuck into her stomach.
Her suspected killer was found sitting on a nearby train track and was arrested.
Police said they had little doubt that she was stabbed by 20-year-old Maurice M., from Düsseldorf in North Rhine-Westphalia. The pair had apparently met on a German internet chat room, Jappy, in May.
Last weekend he made the 600-kilometre journey to see her in Eichwalde, Brandenburg, despite her parents' disapproval, the Tagesspiegel newspaper said on Wednesday.
He apparently developed very strong feelings for Alyssa B., through regular chatting online, classmates of the murdered girl told the Tagesspiegel. “He wanted to marry her,” said one pupil. Another said he had threatened to kill her if she refused.
It is thought, Bild said, that she was trying to break things off with him and when the pair met in person they had a huge fight, said eyewitnesses. “They were really yelling at each other,” a pensioner who was near the attack site told the Tagesspiegel.
A friend of Aylssa B., Willy H., went with the pair into the woods to look out for his classmate. Maurice M. is thought to have hit him over the head with a beer bottle, investigators said. The 15-year-old boy is in hospital.
Maurice M. fled the scene. Officers arrested him at a nearby train station where he was sat on the lines. He had a suicide note in his pocket, the Cottbus state prosecutor said on Tuesday.