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.
A 22-year-old man was arrested in connection with the fatal stabbing of a "bright and popular" teenager on a bus as she travelled to school.
West Midlands Police have launched a manhunt after a 16-year-old girl was fatally stabbed on a bus in Birmingham on her way to school.