Police have said the 16-year-old girl who died after being stabbed on a bus in Birmingham may have been killed in a "totally random attack".
Christina Edkins died from a single knife wound to the chest during the morning rush-hour stabbing, which took place on the A456 Hagley Road in Five Ways in the centre of Birmingham.
A 22-year-old man was arrested on suspicion of her murder and remains in police custody. West Midlands Police have said they have not found a connection between the suspect and the schoolgirl.
Police previously confirmed they are not looking for anyone else in connection with the murder.
A West Midlands Police spokesman said: "At the moment there is no known link between Christina and the 22-year-old man in custody.
"Detectives are exploring lots of possible motives but at the moment this appears to be a totally random attack."
Police added that a murder weapon has not yet been traced. Detectives are still appealing for witnesses to come forward with any information about the suspect's movements between 6am and 1pm on the day of the attack.
Det Supt Richard Baker from Force CID said: "Christina was stabbed on the top deck of the bus...passengers downstairs may not have realised the severity of the attack and left the scene without speaking to police. We need them to contact us as, unknowingly, they may have important information to aid our investigation.
"We believe the attacker continued using the bus network after the attack so passengers on other services may have seen him. We're yet to locate the murder weapon so are keen to re-trace the man's steps in the hope we can find the knife as clearly this is key to the case.
"Yesterday's events were truly tragic and it's extremely important everyone comes forward and speaks to us."
The man, who is wearing a distinctive 'tiger' jacket, is believed to have been on buses in and around the Birmingham area between these times. He is a black male, in his early twenties, and carrying a white bag.
Police said there were around 16 people travelling on the number 9 bus on the Hagley Road when the incident occurred, but only about half have come forward so far.
Officers are urgently asking these passengers to contact them, even if they were sitting downstairs on the bus and didn't witness the stabbing.
Christina was a pupil at Leasowes High School in Halesowen. Police say her family are being supported by specially trained family liaison officers.
The headteacher at Christina's school, Neil Shaw said: "We are trying to come to terms with the tragic loss of Christina and we extend our heartfelt sympathy and deepest condolences to her family and friends.
"Christina was a bright and much loved student who will be greatly missed by all who knew her.
"Educational psychologists and counsellors are in school providing support to students, parents and staff and we will continue to work closely with the police.
"We would like to thank everyone for the many offers of support and warm words of condolence, which means so much at this difficult time."