West Midlands officers say they have spoken to only half the people who were on Birmingham bus during fatal attack

Police want to speak to passengers who were travelling on a bus in Birmingham when 16-year-old Christina Edkins was stabbed to death on her way to school.

In what they described as an urgent appeal, West Midlands police said they had so far spoken to only about half of the 16 people who were travelling on the No 9 bus along Hagley Road in Halesowen when the attack occurred at about 7.37am on Thursday.

A postmortem examination has shown that Christina died from a single stab wound to the chest. A 22-year-old man is being held on suspicion of murder.

Detective Superintendent Richard Baker said: "We know there are a number of passengers who were on the bus yesterday morning when Christina was attacked while she was sitting upstairs. Some of these passengers were sitting downstairs on the bus and they haven't yet spoken to police. They may not even have known what happened but may have vital evidence and it is crucial they contact us as soon as possible on [the force's non-emergency contact number] 101.

"Yesterday's events were truly tragic and it's extremely important everyone comes forward and speaks to us. We will also be able to signpost them to professional agencies who can help them come to terms with being witness to such a tragedy."

Police and emergency medical crews were called by passengers reporting the attack. Christina, who was on her way to Leasowes high school, was pronounced dead at the scene.

Christina Edkins. Photograph: West Midlands police/PA

The headteacher at the school, Neil Shaw, 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."

A tribute page was set up in her memory on Facebook. Dannie Leigh wrote: "Spent all weekend with the most sweetest girl, and it was only two days ago that we were arranging another catch up r.i.p christina, I love you beautiful." Police said family liaison officers were supporting Christina's family. Counsellors were available to pupils and staff at her school.

After the attack West Midlands police issued a description of the suspect and called in firearms officers and police dogs to help the search. A man was later arrested following a chase. He was reported to have been detained after being seen acting suspiciously near a Morrisons supermarket.

Baker said on Thursday that police were investigating the possibility that the killing was a random attack, adding: "We're looking at a number of motives at the moment."

It emerged that Christina had tweeted a month ago about a man "worrying" her on the bus. She wrote: "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 was killed, she tweeted: "Why can't something go right for once?" Police said they were looking at Christina's social media footprint but added: "At this stage there's nothing to suggest she was targeted."

They said that the suspect was already on the bus when she boarded and that there was "nothing immediately" to suggest that the two knew each other.