A 16-year-old boy arrested in connection with the murder of gap-year student Marsha McDonnell has been detained under the mental health act.
Marsha was killed just yards from her home
Detectives say the teenager is being treated at a secure hospital.
He was arrested at 1500 GMT on Tuesday at his home in Hampton, south-west London.
Police said the boy was currently unfit to be interviewed.
Miss McDonnell, 19, was killed near her home in Hampton, in the early hours of 4 February after a night out with friends.
She had been hit on the head with a blunt instrument.
Following Tuesday's arrest, Scotland Yard said a number of addresses in south-west London are being searched.
Officers investigating Miss McDonnell's death have linked it to two other attacks on teenagers, which have taken place within three miles of where she was killed.
A 17-year-old girl was left with head injuries after being attacked in Walpole Gardens, Strawberry Hill, on 8 January at about 2100 GMT.
She has no memory of being attacked and it was originally assumed she slipped on ice.
An 18-year-old man said he managed to escape unhurt after a hooded attacker had lunged at him with a blunt instrument in Hampton Hill a few weeks after Miss McDonnell's murder.
He was then arrested on 18 March for wasting police time.
But on Tuesday, Scotland Yard said he has been released without charge and his assault is still being linked to investigation into Miss McDonnell's murder.
The 16-year-old is the fourth person arrested in connection with the murder.
No one has yet been charged.