Calmly waiting to get off the bus moments after stabbing schoolgirl to death: Knifeman pictured following fatal attack Suspect, 22, arrested and now being questioned on suspicion of murder Post mortem reveals Christina Edkins died from 'single stab wound to chest' 16 people were on bus at time of killing, but only eight have spoken to police By Paul Bentley and Nick Fagge This is the moment a knifeman waits calmly to make his escape just seconds after killing a teenage girl on the bus taking her to school. The CCTV image shows him standing with his back to the driver and his hands on the door handles. Upstairs, his 16-year-old victim Christina Edkins is dying from a single stab wound to the heart. Suspect: Detectives are asking anyone who saw this man between 6 am and 1pm to contact them, even if they were on the bottom deck of the bus and did not witness the stabbing The man would go on to use a series of buses in the area, possibly while still armed, before returning to the scene and being arrested 100 yards away a few hours later. The image of the man, holding a white bag and wearing a distinctive black hooded top with a tiger motif on the back, was taken on a No 9 bus in Birmingham city centre on Thursday morning. It was released as witnesses and police described the terrifying final moments of Christina's life, and her family began to speak of their "absolute devastation." The schoolgirl, who had previously tweeted fears about being followed by a man on a bus, had been sitting on the top deck in front of the knifeman, who approached her slowly from behind without speaking, stabbed her suddenly in the chest and "casually" walked away. Only one other person was upstairs - a schoolboy who heard her screams and rushed to help. Passengers downstairs also heard her shouts. Some have since spoken of fearing for their lives as the knifeman waited to get off. Christina, who was wearing school uniform and was due to sit an exam that day, had boarded the bus just after 7.30am on Thursday near her home in the city centre. She walked upstairs, taking her seat in front of the man who would kill her minutes later as she made her way to Leasowes Community College in Halesowen, about five miles away. "The bus was in the process of stopping when the attack took place," said Detective Superintendent Richard Baker from West Midlands Police. There was no conversation between him and Christina at all. The suspect calmly walked up behind Christina and stabbed her. It happened really, really quickly. This was not a frenzied attack. He's walked downstairs, waited for the bus to stop and walked off. It was [over] in one or two seconds. The attack has been captured on CCTV. The officer said he believed Christina did not initially realise what had happened to her, before suddenly screaming out. Tributes: Friends lay floral tributes near to where Miss Edkins died. A teenage boy, aged about 16 and from a different school, was sitting at the back of the bus and ran downstairs to tell the driver to call 999. He was sitting behind the attacker, on the back seat. He was in his own world. She has clearly called for help," Mr Baker added. A mother of two came face to face with the killer as he waited to get off. Stephanie Bent would later rush upstairs and try desperately to revive Christina with two other women but had to wait for a few awful moments, fearing for her own life, as the killer stood beside her by the bus doors before escaping. "I knew what he had done and he was stood next to me when he was getting off," she wrote on Facebook. All I could think of was, I hope he gets off this stop so I can go upstairs and help the girl. Yesterday, schoolfriends left floral tributes to Christina near the scene of her death. Christina's parents Jason, 42, and Kathleen, 47, were said to be "absolutely devastated." Distraught: A steady stream of mourners visited the scene where Christina died Shrine: By yesterday afternoon, the pile of flowers and tributes had grown into a shrine to the schoolgirl Her brother Ryan, 18, thanked friends for their support and wrote on Twitter that he "would do anything" to see her again. "It is mad to think I will never see you again," he added. I love you. Four weeks before her death, Christina had written on Twitter: "This man is worrying me on the bus keeps getting up and walking up and down stairs, and sitting in different places." Police, who are yet to recover the knife, believe the attack was random. A 22-year-old man arrested on suspicion of fatally stabbing Christina was last night sectioned under the Mental Health Act. Police said experts had deemed him unfit to be interviewed by them any further. Well loved: Christina was described as 'always happy and smiling' Themrsmaynard wrote: 'r.i.p beautiful.. thoughts are with you and the family♥♥xxx'