A man is being held over the shootings of a father and his seven-year-old daughter in an apparent gangland feud.
Toni-Ann's mother is thought to be in Jamaica
Toni-Ann Byfield and her father Bertram, 41, died at a bedsit in Kensal Green, north west London, on Sunday.
Police believe Toni-Ann, who was shot in the back, saw her father being shot and was killed as she was running away, to stop her identifying the gunman.
A 23-year-old man was arrested on Monday night and was being questioned at a north London police station.
On Tuesday police said Toni-Ann's mother, Christine Richards, had been traced in Jamaica and told of her daughter's death.
The seven-year-old was a ward of court, who had been known to Birmingham Social Services since the end of November 2002, and was staying with her father for the weekend.
An inquiry into social services' actions is under way in Birmingham, while anti-gun campaigners are organising a vigil to be held at the scene of the shooting on Thursday evening.
Toni-Ann had only just arrived in the UK from Jamaica and was due to start primary school this week.
Her father, also known as Tony, had served a nine-year jail sentence on drugs charges and had been living in Harrow Road, the scene of the shooting, for two years.
He had survived a previous attack in 2002 in which he was shot six times.
Following that attack, Robert Grant, 23, was acquitted of attempted murder and possessing a firearm at the Old Bailey on 4 October.
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."
Social services review
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.
Birmingham City Council's social services department, which was responsible for Toni-Ann's care, confirmed it had launched an inquiry into the case.
Peter Hay, strategic director of Social Care and Health, said: "In line with Department of Health guidance, we immediately began a review of the case and are working closely with the Area Child Protection Committee and other agencies who will have known Toni-Ann.
"This review is always conducted when a child in the care of a local
authority has died."
He added: "We offer our deepest condolences to her family and will continue to help police with their inquiries."
Anyone with information about the shooting is asked to call police on 020 8358 0608.