A man is being questioned by police over the murder of a teenage girl who was stabbed to death on a crowded bus on her way to school.
Police are investigating whether "bright and popular" Christina Edkins was targeted in a random attack.
She was killed shortly after she boarded the double decker in Birmingham during the morning rush hour on Thursday.
West Midlands Police launched a manhunt for a fellow passenger who fled the scene in the wake of her murder, involving police dogs and firearms officers. A 22-year-old man was later arrested and detained by police following a short chase, after he was seen acting suspiciously near a Morrison's supermarket. The arrest came after police warned the public to be "vigilant" and heightened the presence of officers across the city and on buses.
A spokesman for the police said: "Today was a terrible day for the West Midlands. Our thoughts remain with Christina's family, friends and first responders. First responders have been left traumatised by the incident just like the witnesses."
The stabbing came a month after Miss Edkins voiced concerns about a fellow passenger on another bus journey.
On February 8 she told friends on Twitter: "This man is worrying me on the bus keeps getting up and walking up and down stairs, and sitting in different place."
The teenager, a pupil at Leasowes High School in Halesowen, West Midlands, was attacked in front of hordes of commuters at around 7.30am. Emergency services were called to Hagley Road, in Birmingham, at 7.37am when the vehicle's driver raised the alarm. But medics who rushed to the scene were unable to save her.
Miss Edkins is not believed to have known her assailant, who was already on the bus when she got on board.
Detective Superintendent Richard Baker, of West Midlands Police, said detectives were investigating the theory that her murder was a random attack and said officers were considering a number of possible motives. They are also considering her social networking footprint as part of their investigation and examining CCTV, while specialist forensic investigators were brought in to comb the murder scene.
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