On the eve of the first anniversary of the 21-year-old's death, detectives said they were still looking for a set of keys with key fobs and a "Forever Friends" key ring, believed to belong to Ms Wylde.

A photo of her four-year-old daughter was on a separate key-ring.

The murder hunt began after the single mother was found dead in her home at Torryburn Road, in the city's Barmulloch area, last November.

Detective Chief Inspector John Sherry said: "We discovered that the keys were missing from the house and we believe that after he killed her he took the keys and locked the door as he left.

"Despite an extensive search of the house and questioning hundreds of Tracey's friends and associates in the past year, we have been unable to locate these keys.

The way in which the keys were assembled, using the key fobs makes the set highly unusual," DCI Sherry said.

Key clue

There are five keys on the ring - two mortice keys with red and green plastic covers, two Yale keys and an Ingersoll key.

They are attached to two fobs, one of which is rectangular and features Tracey's name in vertical letters.

Police said Tracey's involvement in prostitution also remained central to the inquiry.

DCI Sherry added: "The vital person we wish to trace and identify remains a man aged between 20 and 45, who was filmed on security video walking with Tracey in the Wellington Street in Glasgow city centre about 3.30 am on Monday, 24 November last year."

"Prostitutes themselves have given us a vast amount of information and the men - or pimps - have also come forward".

The inquiry, which has so far cost £1m, has resulted in 3,000 people interviewed and 2,500 DNA samples taken.