Police have said that the disappearance of three-year-old Mikaeel Kular may be the result of a criminal act.
The young boy went missing from his home on Wednesday 15 January. His mother, Rosdeep Kular, 33, put him to bed at around 9pm on Wednesday, only to find his room empty when she checked at around 7.15am the next morning.
It was originally suggested that the boy may have left the family's flat in Edinburgh himself.
In a police update, Assistant Chief Constable Malcolm Graham, of Police Scotland, has since reported that investigators are also looking into the possibility of criminality.
"It is, however, entirely possible that Mikaeel has become subject of a criminal act and our investigation has been established as a twin track to ensure that eventuality is being, and will be, fully explored," he said.
"It is of grave concern that Mikaeel has been missing for nearly 48 hours, since he was last seen on Wednesday night by his mum, and all of the work we've been doing, we've been unable to confirm any sightings of Mikaeel.
"As a result, our operations and our investigations are continuing in tandem but we must make sure we examine fully every possibility as to what might have happened."
"I again make a plea if anybody out there has information which is going to assist us in better understand the circumstances in which Mikaeel has gone missing, that they make contact with the police."
Superintendent Liz McAinsh said there had been a "positive response" to the appeal for information in relation to the potential sighting.
"We have calls that we are following up and, once we know more about that that we can report to you, we will do that," she said.
"You've all seen today the huge public response that we have had to this, and I would like to reiterate the thanks to everybody that has come out in our search for Mikael.
"That search will continue until we have any circumstances that dictate otherwise."
All immediate family members have been traced and spoken to the police in connection with the investigation.
"It has been extremely helpful to understand all the circumstances that have led up to his disappearance," Graham said.
He added: "We have established a timeline of Mikaeel's movements going back for sometime prior to when we can establish that he was last seen in the house on Wednesday night.
"Now it is important for us that anybody who has got information about having seen Mikaeel comes forward and gives that to us.
"There is a timeline going back of when he has been seen by different people, but I am not able to go into the detail of that at the moment."
"This is an extremely significant inquiry. I think everyone can get a sense of the urgency and scale of the resource that Police Scotland is able to put towards establishing the circumstances," he added.
Hundreds of volunteers joined the search of the area for any sign of the missing boy. Volunteers including fire fighters and mothers pushing prams, helped scour the foreshore at nearby Cramond.
Specialist teams have been deployed from across the country to help search, investigation and community support work. Mounted officers, dogs, other emergency services and local organisations were also taking part in the investigation near and at the family home in Ferry Gait Crescent.
Four forensic officers in white suits and masks were seen entering the flat.
Officers said earlier that they had spoken to Mikaeel's father and all of the boy's relatives are said to have been very helpful to the police with their enquiries.
Police had already announced that officers are investigating a "potential sighting'' of a child matching Mikaeel's description and wearing a cap.
Members of the public reported seeing a boy who was "small in build and alone'' in West Pilton Gardens, Edinburgh, at around 8.30am yesterday.
A missing person appeal is to be displayed on digital billboards across the UK, the charity Missing People confirmed.
The little boy, who has two sisters and two brothers, usually sleeps in the same room as his twin sister but was alone on Wednesday night.
His mother is being supported by police, her family and friends, but is said to be "very distraught."
Police have activated the UK-wide Child Rescue Alert system, which is usually reserved for abduction and kidnap cases.
Anyone with information relating to the sighting or the boy's disappearance is asked to contact the dedicated telephone number 0300 200 0200.