A man who fatally stabbed a 16-year-old schoolgirl on a bus carried out the knife attack and then “calmly” waited to get off the vehicle, a detective has said.

Christina Edkins was killed on a double-decker in Hagley Road, Birmingham, during the morning rush-hour on Thursday.

The teenager had been sitting in front of her attacker on the top deck when he struck as the bus was slowing down. Postmortem tests have revealed she died from a single stab wound to the chest.

Detective Superintendent Richard Baker, of West Midlands Police, said after the attack: “He walked down the stairs and calmly waited to get off the bus.”

Police have released a picture of the suspected killer, a black man in his early 20s, wearing a distinctive jacket with a picture of a tiger on the back. They believe he used buses in and around Birmingham around the time of the attack, and have appealed for anyone who saw him between 6am and 1pm.

They also called on any remaining passengers from the number nine bus to speak to them about what happened.

Det Supt Baker said: “Even if you think the information you have is not relevant or will not assist us, please let us make that assessment.”

He added: “We know from witness accounts that, although some screaming was heard immediately after the incident, initially people thought it was just teenagers messing around on the bus.”

Yesterday a 22-year-old man arrested near the scene was questioned by detectives on suspicion of murder, but hours later police said the man has been sectioned under the Mental Health Act.

A spokesman for West Midlands Police said the man will now be sent to a secure facility after the latest assessment by mental health professionals deemed him unfit to be interviewed any further by detectives.

Police said the suspect had been regularly assessed by medical experts throughout his time in police custody.

A woman claiming to have witnessed the incident on the bus said she waited for the girl’s attacker to get off the vehicle before performing CPR on the teenager.