A 22-year-old man arrested on suspicion of fatally stabbing schoolgirl Christina Edkins on a bus in Birmingham has been sectioned under the Mental Health Act, West Midlands Police said.
The 16-year-old was stabbed on a bus on Hagley Road on Thursday, while on her way to school in Halesowen.
A post mortem examination revealed she died from a single stab wound to the chest.
West Midlands Police said the suspect would now be sent to a secure facility.
Officers said an assessment by mental health professionals deemed him unfit to be interviewed any further by detectives.
They said the 22-year-old, a British national with links to Walsall, was not previously known to mental health units in the area and has no known link to the teenager.
Christina is understood to have boarded the number 9 bus minutes before the attack, which happened at about 07:40 GMT.
Fellow pupils, parents and staff at Leasowes High School in Halesowen have laid more than 100 bunches of flowers outside the school gates in her memory.