Victorian emergency crews are scouring through bushland into the night for an autistic boy who went missing in Melbourne's outer east.
William Wall, 14, left to go for his daily walk in the Yarra Ranges about 6.45am on Tuesday.
He told his family he would be gone for 15 minutes but never returned, police said.
William, who has high functioning autism, left home without his mobile phone. His family and police have serious concerns for his welfare.
An air and ground search involving local police and the State Emergency Service has been initiated to find him.
"We've got multiple agencies out here ... if we need to go into the night we definitely will," Acting Sergeant Jordan Kavanagh told reporters on Tuesday afternoon.
"The air wing have capabilities to detect heat during the night."
Acting Sgt Kavanagh said the Warburton Rail Trail, which runs about 40km from Lilydale to Warburton, covers difficult terrain.
Crews were focusing on the O'Shannassy Aqueduct Trail above Warburton after a possible sighting from a "credible witness".
"The witness has described an upset male matching the description," Acting Sgt Kavanagh said.
Acting Sgt Kavanagh said William's "upset" family was out helping to search for him.
The teenager is described as 177cm tall, with a lean build, mousy brown hair and a lazy right eye.
His family said he was wearing a dark hooded jumper, dark-coloured track pants, black runners and had a CamelBak hydration backpack.
Anyone who sees William is advised to contact police and approach him in a calm manner.
