Detectives have launched a manhunt after a schoolgirl was stabbed to death on a bus during the morning rush hour.
The 16-year-old was killed on the crowded double decker as she made her way to school.
Officers are searching for a black man in his late teens or early 20s and have warned the public to be "vigilant".
A man was detained by police in connection with the stabbing but later released without arrest.
Detectives began a murder investigation as officers broke the news to the family of victim, who has yet to be formally identified.
Detective Superintendent Richard Baker, of West Midlands Police, said: "As you can appreciate the family are extremely distressed and distraught by the news of today's events.
"In relation to the offender, we are currently actively undergoing a manhunt operation in Birmingham. We are utilising every specialist officer, local policing officers and investigators in order to identify, locate and arrest the offender who is currently outstanding."
He said police were investigating the possibility the murder was a random attack, adding: "We're looking at a number of motives at the moment."
The suspect is believed to have worn dark-coloured trousers and a dark-coloured hooded top with a distinctive tiger or leopard motif on the back. Witnessed said he may have been carrying a small white bag or have had a bandage around his right hand.
The area was cordoned off on Thursday morning and and officers remained at the scene on Hagley Road, in Birmingham. The street, leading out of the city, was closed in the wake of the stabbing and is expected to stay shut for the rest of the day.