The murder of a seven-year-old girl and a 41-year-old man is to be featured on the BBC's Crimewatch programme.
Toni-Ann with the man she thought was her father, Bertram Byfield
Toni-Ann Byfield from Birmingham was shot alongside Bertram Byfield, 41, just over a year ago at a flat in Harlesden, north west London.
Police believe Mr Byfield, a convicted drug dealer, may have been killed over a drug debt.
A reward of £25,000 is being offered for information that leads to those responsible for the murders.
Toni-Ann had been allowed by social services staff in Birmingham to visit Mr Byfield in London. He was not her biological father, but the child regarded him as such.
Detectives think Mr Byfield had links to the British Link Up Crew, a group of Jamaican crack dealers running cocaine between the UK and Jamaica.
Det Ch Insp Neil Basu said Mr Byfield, formerly known as Anthony Pinnock, was owed £25,000.
"We are investigating the possibility that Mr. Byfield was killed over a drug debt that he previously tried to collect.
"This day last year, a seven-year-old girl was shot dead in front of the man she loved as a father, only moments before he too had been fatally wounded.
"I would like the public to bear in mind that the persons ultimately responsible for her death are the person who pulled the trigger that morning and the person who ordered it to happen.
"I want to bring those people to justice."
Detectives believe Toni-Ann, who was shot in the back, may have been killed to stop her identifying the gunman.
The girl had been raised in Jamaica but arrived in the UK in June 2000 to stay with relatives in Birmingham.
A full reconstruction of the case will be shown on Crimewatch on BBC 1 at 2100 BST on Tuesday.