Forensic tests have revealed the killer was wearing a pair of Lonsdale trainers at the time of the murder.
DCI Martin Kinchin added: “It is likely that the offender would have disposed of them as they were likely to be blood stained.
“They are a budget trainer which is grey or white in colour. We don’t know the size, but we know that it was a trainer which was made in small numbers between 2006 and 2011.
“It can only be sold in Sports Direct stores or online. They are available across Europe. The UK is the major market for them.”
Local Sports Direct stores include Kettering, Corby, Bedford, Northampton and Milton Keynes. A Sports Direct store in Wellingborough recently closed.