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".
South Wales Police are appealing for information over the whereabouts of a man wanted in connection with the murder of a Cardiff teenager.