Rosdeep Kular was detained for questioning after her son Mikaeel Kular's body was found and a house in Fife, north of Edinburgh, believed to be that of her sister, was sealed by police last night for forensics officers to conduct a probe.
Mikaeel disappeared after he was put to bed by his 33-year-old mother at their home in Ferry Gait Crescent in Edinburgh on Wednesday night. Hundreds of local residents and volunteers had been searching for him since then.
Police have confirmed that he had not attended Flora Stevenson Nursery since Christmas and said a "criminal act" was possible.
Rosdeep's mother, Harjinder Kular, yesterday paid tribute to the volunteers who had joined the search.
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Another cordon was strung around the woods with access to a private road around the back blocked by police.
A neighbour told BBC that Rosdeep and her five children lived at the address until 18 months ago and Mikaeel and his twin Ashika were born while they lived at the house.
Under Scottish police procedures, Rosdeep, a single parent who has four more children, can be detained for an initial 12 hours for questioning without being formally arrested or charged and police can apply for a further 12-hour extension.
Mikaeel's father, who is understood to be estranged from his mother, has been questioned by police.