Christina Edkins, the schoolgirl who was killed on a bus on the way to school, died of a "single stab wound to the chest" police have confirmed.
The 16-year-old was knifed as she travelled along the Hagley Road in Edgbaston on her way to Leasowes High School.
Detectives are appealing for any passengers who were travelling on the bus to contact them.
Sixteen people are believed to have been travelling on the bus when the stabbing occurred, but only half have so far spoken to police.
Detective Superintendent Richard Baker, of West Midlands Police, said: "We know there are a number of passengers who were on the bus yesterday morning when Christina was attacked while she was sitting upstairs.
"Some of these passengers were sitting downstairs on the bus and they haven't yet spoken to police.
Police have arrested a 22-year-old man on suspicion of murder and he was still in custody this morning.
He was held near a Morrison's supermarket following a manhunt launched after the killing on a crowded double-decker bus at around 7.30am as Christina made her way to school.
They said he was already on the bus when Christina got on board. Officers said there was "nothing immediately" to suggest the pair knew each other. They are not looking for anyone else in connection with the murder.
Det supt Richard Baker from Force CID said: "This is a tragic case and the victim's family are devastated.
"We are still anxious to hear from anyone with information or who may have witnessed the incident this morning to speak to us via the dedicated hotline 0800 096 0095."