Aamir Siddiqi, 17, was stabbed at his home in Roath in 2010 and two men, Jason Richards and Ben Hope, were jailed for life in 2013.
Mohammed Ali Ege, originally from Cardiff, was arrested in India in 2011 accused of conspiracy to commit murder.
But he escaped police custody in India last week.
Mr Ege, who is also accused of passport and identity forgery, was awaiting extradition but escaped after being taken to a court hearing.
Authorities confirmed he escaped from a railway station washroom in New Delhi.
Det Ch Insp Ceri Hughes said Mr Siddiqi's family "remain resilient and continue to receive our full support".
Mr Ege is from the Riverside area of Cardiff, but he also has connections with people in surrounding areas.
DCI Hughes added: "We believe someone in this country knows something about Mohammed Ali Ege's whereabouts, and urge such people to contact us".

The aim is to cut through the jargon and present election news in a different way.
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Follow our reporter Steffan Messenger on Facebook, where he'll be posting all our #iNeverKnew coverage and focusing on the issues you feel passionately about - your cash, your career, your future.
And check out our guide to what the parties say they'll do for young people here.

Six-month-old Molly-Mae Wotherspoon was fatally attacked by an American pit bull named Bruiser at a house in Daventry in October 2014.
Mother Claire Riley, 23, and grandmother Susan Aucott, 55, are currently in prison in connection with the death.
Northamptonshire Police said it had "overhauled" its dangerous dogs policy.
LIVE: For more on this and other Northamptonshire stories
The serious case review highlighted a number of failings.
The report states Riley took Bruiser - one of two dogs in her home - to a vet in January 2014 when Riley was pregnant.
The vet voiced his concerns about the fact the animal was an illegal breed to the RSPCA, which made a referral to the police.
The report said the force failed to act on these concerns.
Det Supt Steve Lingley, head of safeguarding at Northamptonshire Police, welcome the report's findings on what he said was "a very challenging case on a number of levels".
"Within weeks of the tragedy," he said, "Northamptonshire Police had carried out a complete overhaul of its response policy to dangerous dogs."
Keith Makin, chairman of the  Northamptonshire Safeguarding Children Board (NSCB), recommended "greater curiosity" about family pets amongst health workers.
He also called for "all police officers to be made aware through safeguarding training of the dangers which aggressive dogs pose to young children".
The report stated: "The dog was so fierce that a vet had earlier refused to examine it for an illness unless it was first fully sedated."
It also said that while Riley was "fully aware that her own mother had alcohol-related problems" she went out for evenings leaving her in "sole charge of the baby while the dogs were in the house".
The report concluded "although some poor practice has been highlighted" by the board's inquiries, there was nothing which suggested "any single professional could, or should, have prevented" Molly-Mae's death.
South Wales Police are appealing for information over the whereabouts of a man wanted in connection with the murder of a Cardiff teenager.
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Throughout the election campaign BBC Wales News is answering young voters's questions and reflecting their views through our interactive #iNeverKnew project.
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Police failed to act on concerns raised about an illegal dog before it killed a baby girl, a report has found.