Specially trained police officers are to interview pupils at a primary school where a boy collapsed suffering from the effects of the drug methadone.
Merkinch Primary School, where the pupil collapsed
The pupil, aged seven, fell ill in class at Merkinch Primary, Inverness, and was treated in Raigmore Hospital.
Northern Constabulary plan to speak to children who may have seen him take the methadone, possibly in the school playground at lunchtime.
The substance is not believed to have come from the boy's home.
Highland Council's social work department and police have begun a joint investigation into how the youngster got the methadone.
Officers have interviewed teachers, janitorial staff and playground supervisors.
Police said specially trained child protection officers would work closely with the social workers to interview a number of pupils who may have been in the playground at the time the boy may have taken the drug.
Chief Inspector Ian Cox said: "We are obviously trying to establish, at this stage, where the substance came from.
"Due to the risk which was posed to the child's safety this incident is being treated as a priority inquiry.
"We are looking at a number of positive lines of inquiry and these must all be bottomed out. We will continue our investigation until we establish the source."
He added: "There is absolutely no indication that this substance has come from the child's home."
Hospital blood tests confirmed the boy had methadone in his system.
The pupil was discharged from hospital on Tuesday and is said to be making a good recovery.
Chief Inspector Ian Cox is trying to trace the source of the drug
A council spokesman said: "The pupil seemed to be drowsy in the class and the teacher went up to him and next thing he collapsed.
"She got the ambulance to come and staff had no idea what was up with him but he clearly wasn't well. We have had confirmation that he had methadone in his system.
"The local education and social work authorities are working with their police and health colleagues to find out how this happened."
Meanwhile, the school's head teacher Sheena Morrison warned pupils at assembly on Thursday about the dangers of drugs.
Later she said: "One of the themes in our school that we bring up is that a child should keep themselves safe.
"In light of what has happened we will be underlining to the children the need to keep themselves safe in terms of drugs."
Peter Corbett, Highland councillor for the Merkinch area of Inverness, said the community was shocked.
He said: "I'm really awfully glad the youngster has made a full recovery and he is fit and well at home."
Mr Corbett said the council has a "stringent" drugs policy and commended the school's reaction.
Methadone is used as a substitute for heroin
He added: "The question here is how did the youngster get hold of methadone and that is what the investigation with police and social work is."
Mr Corbett described methadone as a dangerous substance and referred to the death before Christmas of East Lothian two-year-old Derek Doran, who allegedly drank the heroin substitute.
His parents, Lisa Dodds, 25, and Derek Doran Snr, 21, of East Lothian, appeared in court in May charged with his murder.
Mr Corbett said Merkinch did not have any worse a problem with drugs than any other area of Inverness or Scotland.
He said: "It is really worrying that a youngster can get hold of methadone. I do understand he did not get it from his own house.
"Where he got it from we don't know.
"Merkinch is the same as every area - there is not an area that does not have a drug problem."