A 16-year-old girl stabbed to death on a bus in Birmingham's morning rush hour has been named by West Midlands Police as Christina Edkins.
A 22-year-old man was chased and arrested near a Morrisons supermarket in the city after he was spotted acting suspiciously, police said.
Police had launched a manhunt after Christina, 16, was killed on the crowded double-decker at about 7.30am today as she made her way to school.
Police said the suspect was already on the bus when Christina got on board.
Christina is a student at Leasowes High School in Halesowen, West Midlands Police said.
She also posted on the micro-blogging site that she had concerns about a fellow bus passenger.
Just last month, on the morning of February 8, @ChristinaEdkins tweeted about a man who was "worrying" her on a bus journey.
She said: "This man is worrying me on the bus keeps getting up and walking up and down stairs, and sitting in different place:|"
Less than 24 hours before she died, Christina tweeted at 1.28pm on March 6: "Don't need anyone to talk to about how I feel just need someone to be lazy with me and watch films and make me laugh."
She also posted pictures of herself and her school pals in their uniforms during a French lesson.
On February 27 she tweeted a picture of her maths texts books and said: "Having to revise this all for tomorrow."
On February 23 she tweeted: "When I'm on bus and @KatieEllizabeth sends me something funny and I burst out laughing and people just stare at me, hahaha opsyy."
On the same day she tweeted: "Eughh hate catching buses, can't wait to start driving."
Neil Shaw, the headteacher of Leasowes High School, said: "Christina was a bright and popular student much loved by staff and students alike.
"We are deeply saddened to hear this tragic news and our thoughts and hearts go out to her family and everyone who knew her.
"The school is working closely with the police and a team of counsellors to provide support to our pupils and staff."
A man was earlier held by police in connection with the stabbing but he has now been released without arrest.
Christina's family have been informed by police and were being supported by family liaison officers.
Detective Superintendent Richard Baker said: "As you can appreciate the family are extremely distressed and distraught by the news of today's events.
"Few of us can imagine what they are going through."
Officers said there was "nothing immediately" to suggest the pair knew each other.
They are not looking for anyone else in connection with the murder.
The city centre attack comes just a few months after two big issue sellers were stabbed to death in broad daylight in the middle of the city centre.
In May 2001 16-year-old Rosemarie Ross was stabbed to death as she sunbathed in the sunshine in Centenary Square, Birmingham.