Detective Inspector Richard Baker explains that 16 people were on the bus when Christina died.
He has spoken to eight of them, he would like to speak to the other eight about what they saw and heard.
The police believe people may have heard screams on the bus but mistook that for people 'messing around'. They would like anyone who thinks they heard or saw anything to contact them on their dedicated hotline: 0800 096 0095.
West Midlands Police investigating the murder of 16-year-old Christina Edkins have released a CCTV image of a man in the hope it will jog the memories of people that may have seen him.
The image was taken shortly after the attack, which shows him leaving the bus.
They arrested a 22-year-old man near to the Morrisons supermarket yesterday lunchtime.
Police are keen to talk to anyone who saw him.
Police investigating the fatal stabbing of 16-year-old Christina Edkins have confirmed they are not looking for anyone else in relation to the incident.
It follows the release of a CCTV image of the man they arrested. Police are asking people in Birmingham who saw the man yesterday morning to come forward and tell them what they saw.
Police investigating the death of a teenage girl are asking people in Birmingham to let them know if they saw this man yesterday morning.
The man, who is wearing a 'tiger' jacket, is believed to have been on the buses in the Birmingham area between 6am and 1pm.
A book of condolences is being kept at a church in Lapal for the teenage girl stabbed in Birmingham yesterday.
16 year-old Christina Edkins died on a bus in rush hour yesterday morning.
A post-mortem examination revealed that 16-year-old Christina Edkins died from a single stab wound to the chest, West Midlands Police said.