Passengers on the lower deck of a bus where a girl was stabbed to death heard screaming but thought it was teenagers messing about, police have said.
Officers said they believed the attack on Christina Edkins during Thursday morning's rush hour in Birmingham appeared to be "random and senseless".
A 22-year-old man has been arrested on suspicion of murdering the 16-year-old who was attacked on the bus's top deck.
Police said they were still searching for the murder weapon.
Officers have until 01:00 GMT on Saturday either to charge or release the man they have in custody.
Christina was stabbed once in the chest on the bus on Hagley Road while on her way to school in Halesowen.
Earlier, police said they "urgently" needed to speak to all of the 16 people who were on the bus at the time of the attack, at about 07:30 GMT.
In a press conference, Det Supt Richard Baker said there had been three people on the top deck of the bus at the time of the attack - Christina, her attacker and another man, who has been identified.
Mr Baker said he believed Christina's attacker may have travelled on buses around the Birmingham area after the stabbing.
Counsellors have been brought into Leasowes High School, where Christina was a pupil, to support pupils and staff.
Throughout the day, pupils and people living nearby have been leaving flowers, soft toys and balloons outside the school gates.
The school said it thanked "everyone for many offers of support" and extended heartfelt sympathy to Christina's family.
A book of remembrance has been opened at nearby St Peter's Church, in Lapal, Halesowen, where people can go to light a candle.
Several supermarkets in Halesowen have started collections for Christina's family.