Four years on from the unsolved murder of a Greenwich University student in Penge, police say a mystery telephone number could hold vital clues.

Samuel Guidera, 24, died in hospital after he was stabbed him in the heart and mugged near Penge East Station on February 12 2011.

The detective leading the investigation has released details of a telephone number inputted into Samuel’s phone moments before he was murdered.

Detective Chief Inspector Chris Jones and his team are working on a theory that his killer or killers may have got his attention because they wanted to use his mobile phone.

Samuel was found collapsed in Bailey Place at 9.55pm after travelling on the 9.17pm Bickley to Penge East train.

Officers believe Samuel arrived at the station at around 9.30pm before crossing the railway footbridge to the Newlands Park exit en route to meet his girlfriend.

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Samuel Guidera before he left Penge East station 

The history and politics student, who had spent the day watching football with friends, was last seen at the bus stop on Bailey Place at 9.38pm.

At 9.48pm, Samuel’s phone was used to dial 0740 477 6433, a number not connected to Samuel or anyone he knew.

Seconds later, Samuel tried to call 999 but dialled 9999 and the call did not connect.

Police have made extensive enquiries and established the number belongs to Lyca Mobile and a Pay-As-You-Go Sim, which has never been activated.

Officers want to speak to anyone with information about the number, no matter how small, or anyone with a Lyca Mobile Sim with a similar number ending in different digits.

DCI Jones said: "Four years is a long time for Samuel’s family and friends to wait for justice and it is also long enough for alliances to change.

"Someone out there has information which can help us to identify Samuel’s attackers and I would appeal to them to contact the police."

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Samuel's parents Sarah and Chris Guidera (Photo: BBC Crimewatch)

Samuel’s parents, Sarah and Chris Guidera, had previously said following his death, they had started a life sentence from which, they would never be released.

They said: "We still feel a sense of anger and all we can do is continue to cope with each day as it comes, with the help of medication, family, friends, whatever it takes.

"As parents we have seen the devastation and heartbreak it has caused Sam's brother and sister, other family members and friends.

"We just want somebody out there who knows something that could help in the apprehending of the killers to come forward so they do not have the opportunity to take another person's life and destroy another family."

His friend Claire Smith said: "I feel so privileged to have been able to say "I love you" and hear him say it back.

"Now we direct those words to a hole in the ground and hear nothing but silence in response."

Anyone with information should call 020 8345 3734; or Crimestoppers anonymously on 0800 555 111.