Police can find no motive for teenager's murder

Detectives said yesterday a popular young student beaten to death yards from her home might have been the victim of a random attack by a mentally disturbed killer.

Officers said the motive for the attack on Marsha McDonnell remained a "complete mystery" but revealed someone had beaten her from behind with three deliberate blows to the head.

Ms McDonnell, 19, may not have seen her killer. She was attacked as she returned home from the cinema in the early hours of Tuesday.

Police are also exploring the possibility it could have been an attempted robbery or a sexual assault that went wrong. Her bag, telephone and cash were left with her.

They have looked into her private life and believe there is nothing to provide a link with the attack. There was no evidence of a sexual assault.

Ms McDonnell, who was on a gap year, was found in the street at Hampton, south-west London. She was attacked after getting off a number 111 bus on her way home.

Footage from a security camera on the bus suggested she had not been followed. Detective Chief Superintendent Dave Cook, who is heading the murder investigation, said: "The motive is a complete mystery. We don't know what it was. It wasn't a frenzied attack, there were quite deliberate blows to the head.

"Early indications are that she was attacked from behind and she was struck three times on the head."

He said the area had been searched thoroughly but there was no sign of a weapon. Detectives would be speaking with colleagues working on the inquiry into the murder of the American artist Margaret Muller, killed in an east London park earlier this week.

"We are looking at an isolated incident, violent, unprovoked – how else can you describe an attack on a 19-year-old girl walking home?" he said. Mr Cook said his team had been busy reconstructing Ms McDonnell's final moments.