A seven-year-old girl was shot in the back as she fled her father's killer, police believe.
Toni-Ann's mother is thought to be in Jamaica
Toni-Ann Byfield and her father Bertram, 41, died following the shooting at a bedsit in Harrow Road, Kensal Green, north west London, on Sunday.
Detectives believe Toni-Ann witnessed her father being shot and was killed to stop her identifying the gunman.
A 23-year-old man was arrested in connection with the shootings on Monday night and was being questioned at a north London police station.
Interpol is trying to track down the girl's mother who is thought to be in Jamaica.
Toni-Ann was a ward of court who was staying with her father for the weekend.
She had only just arrived in the UK from Jamaica and was due to start school on Monday.
Her father, also known as Tony, had served a nine-year jail sentence on drugs charges and had been living in Harrow Road for two years.
He had survived a previous attack in 2002 in which he was shot six times.
Although police do not know who killed the father and daughter, the investigation has been taken over by Operation Trident, which deals with gun crime in London's black community.
But Detective Chief Superintendent John Coles said: "I can't rule out that the killers are white or any other group."
It is the first time Operation Trident has investigated the death of a child.
They are stressing that the suspect or suspects might be very different from the usual criminals they deal with.
Mr Coles said: "Mr Byfield was shot several times and his daughter once in the back, I assume that she was trying to run away from the gunman.
"It is without doubt the most evil and despicable crime I have come across in all my service and it shows the lengths some people will go to to protect their identity after committing a murder."
The father and daughter were found still alive at the bedsit just after midnight on Sunday, after neighbours reported hearing shots and a scream - thought to be Toni-Ann.
They died before they reached hospital.
Anyone with information about the shooting is asked to call police on 020 8358 0608.