Police hunting the killer of schoolboy Rory Blackhall are urgently trying to identify a man who was near the spot where the 11-year-old was last seen.
Police said the man was wearing a distinctive hat and jacket
Rory's body was found in woodland, three days after he failed to turn up at Meldrum Primary in Livingston.
A post mortem examination revealed Rory died from asphyxiation.
Detectives said they had become "increasingly anxious" to trace a man seen in a trapper hat and a black bomber jacket.
Speaking at a news conference, Detective Inspector Tom Martin, of Lothian and Borders Police, said officers had received no calls or information regarding the sighting of the unknown man.
He said: "At about 0830 BST on the day before Rory went missing, a member of the public was walking his child to school along the pathway which leads from the underpass where Rory was dropped off by his mother.
"At this time, the witness's attention was drawn to rustling in the wooded area at the side of the pathway and a man emerged from the bushes and trees beside him near to the electricity substation.
"The man had to push aside some large bushes to exit the wood."
Police, who did not comment on whether the man was a suspect in the murder case, said he was unaccompanied and was not seen to be carrying anything.
Mr Martin said the witness, who regularly used the same route to walk his child to school, had never seen the man before.
He added: "He made no attempt to acknowledge the witness, which he also regarded as unusual for people in that area.
"He appeared to be completely out of place for the area and the witness was actually concerned about his presence there."
Rory's body was found three days after he failed to turn up at school
Mr Martin said the man was seen with the flaps of his trapper hat down covering his neck and ears.
"The witness thought this was particularly unusual given the weather conditions at the time."
Mr Martin said the unknown man was described as 38 to 40 years old and 5ft 8in with a "gaunt, very pale appearance".
He was also described as having "quite a pointed nose" and had light or grey stubble on his chin.
His black bomber jacket was zipped up to the neck and he also wore black jeans and black boots or shoes.
The trapper hat was dark green and made of waterproof nylon.
Mr Martin said: "We would ask members of the public to closely consider both the description of the man along with the distinctive hat and jacket.
"It may be that people might recognise the description of the man but wearing only the distinctive hat, not the jacket, or, indeed, they recognise the description of the man with only the jacket and not the hat."
The police officer added: "It should also be borne in mind that, while the witness saw the man with the flaps of the hat pulled down on this occasion, others may have seen the man with the hat fastened in the upright position.
Rory's family laid flowers near the spot where his body was found
"We would again ask that anyone who knows or believes they may know this man, or who has seen a man fitting this description and wearing either the hat or jacket, or both, to contact the police as a matter of urgency."
Saturday's appeal came following a mini-reconstruction on Thursday of Rory's last known movements.
Following the reconstruction, police said they now traced four people - two males drinking beer on Nellburn Path on the day Rory was last seen, and two other males who were at a campfire on Dechmont Law last Saturday.
Only the man wearing the bomber jacket remains unidentified.
On Friday, Rory's family laid flowers near the spot where his body was found.
Russell Blackhall, 42, his wife Michelle, 41, and Rory's 14-year-old brother Conal spent 15 minutes looking at other tributes at the scene in Livingston.
Rory's mother cried as she read messages to the 11-year-old which had been left by other people.
She left a bunch of thistles and lavender, tied with a tartan ribbon.
The card read: "Rory Pory, you are always with us in our thoughts and in our hearts.
"Our wee Rory, we love you."