The assault was filmed and the tape is expected to show that it was the dead 16 year old who attacked the girl’s brother.
The attack on the brother, who is aged 15, was filmed on a mobile telephone and was circulated among youngsters in Örebro on Saturday.
“A film showing her brother is circulating,” said police spokesman Torbjörn Carlson.
“Naturally, we will analyse the film. We need to do that carefully to establish that it really is the brother. Also, it will naturally be linked to this investigation as a possible cause, but I don’t want to speculate on that,” said Carlson.
The film is said to show how the 16 year old beat the girl’s brother bloody. It is also thought to show how a large gang gathered to watch them fighting.
After visiting her brother in hospital the girl went looking for the 16 year old, seeking revenge, the police suspect.
But the notion of revenge was rejected by the girl’s lawyer, Christer Johnsson.
“That’s not the case. There wasn’t any revenge here. There is no motive either – she has not done anything with a view to injuring him,” Johnsson told TT.
The police are still declining to comment on possible motives, but a number of media reports suggest that the killing was gang related.
“We must obviously investigate this gang theory, but I think it’s too early to speculate on that too,” said Carlson.
The boy was was knifed outdoors in Vivalla, a residential area in the town, just after midnight. He was taken to the Örebro university hospital where doctors said his injuries were so serious that his life could not be saved.
A witness and several residents from the area have been questioned by police.
The girl denies murder or manslaughter, according to her lawyer.