A 16-year-old girl who was attacked on rush-hour bus in Birmingham died from a single stab wound to the chest, post-mortem tests have revealed.
Christina Edkins was stabbed upstairs on the bus on Hagley Road while on her way to school in Halesowen on Thursday.
West Midlands Police are continuing to question a 22-year-old man who was arrested near the scene of the stabbing a few hours later.
Police are investigating whether it was a random attack.
They say they are not looking for anyone else in connection with the killing.
The force said about 16 people were on the bus at the time of the attack, but they had only spoken to half of them.
They urged the other passengers to contact them urgently, even if they did not see the stabbing and had been sitting downstairs.
Officers have released a CCTV image of a man wearing a jacket with a tiger design on the back and have urged anyone who saw him on Thursday between 0600 GMT and 1300 GMT to contact them.
West Midlands Police said extra officers were patrolling buses on Friday to reassure passengers.
Counsellors have been brought in to support pupils and staff at Leasowes High School, where Christina was a pupil.
Flowers, soft toys and balloons have been left outside the school.
A book of remembrance has been opened at nearby St Peter's Church, in Lapal, Halesowen, where people can go to light a candle.
Christina's head teacher Neil Shaw described her as a "bright and popular student" who was "much loved by staff and students".
'Distressed and distraught'
The Reverend Mike Rutter, the vicar of St Peter's said: "It's a terrible tragedy and many in our community will be affected.
"Our thoughts and prayers are with Christina's family and friends at this difficult time."
Det Supt Richard Baker said the teenager's family were distressed and distraught.
Halesowen and Rowley Regis MP James Morris said he would be meeting officials of National Express later to discuss bus safety.
He added: "The whole community is in shock here in Halesowen about this tragedy."
Police said they would be checking Christina's profiles on social media websites as part of the investigation, but there was nothing to suggest she was being targeted.
She is understood to have boarded the bus minutes before the attack, which happened at about 07:40 GMT.
The driver raised the alarm and attempts were made to revive the teenager, but she died at the scene.
Police have urged any further witnesses to contact them.
The 22-year-old was arrested near the Morrisons supermarket at Five Ways at about 12:30 GMT.
Ch Insp Kerry Blakeman, head of operations for Safer Travel, an organisation run by West Midlands Police, British Transport Police and Centro, said Birmingham's buses were safe to travel on.
He said: "If you feel uncomfortable with anything, contact the driver.
"There is a reporting mechanism between the buses and the police and officers can be contacted immediately."