Two 18-year-olds have died following a suspected carbon monoxide leak in Castlerock, County Londonderry.
Another boy was taken from the holiday apartment at Tunnel Brae Court to the Causeway Hospital in Coleraine where he is said to be in a stable condition.
It is thought a relative of one of the young men raised the alarm at about 2130 BST on Tuesday after phoning him.
The boys who died were Aaron Davidson, of Fernagh Road, and Neil McFerran, of Kings Walk, both in Newtownabbey.
It is understood that Aaron was awaiting his A-level results and Neil his A/S-level results.
Health and safety officials have been checking gas appliances inside the flat to trace a leak, and the apartment block has been evacuated while tests continue.
John McPoland from the Ambulance Service said: "All the signs and symptoms were that it was carbon monoxide poisoning.
"When the Fire Service arrived, they used some of their equipment and they confirmed to the paramedics that there were high levels of carbon monoxide in the room."
Mr McPoland said it was too soon to determine the source of the carbon monoxide.
"The thing about carbon monoxide poisoning is that the carbon monoxide itself is odourless, colourless, tasteless so it's a silent killer," he said.
"You wouldn't know what's actually happening. You would just become very tired, weak and disorientated."
On Wednesday, the headmaster of the teenagers' school, Glengormley High, said they were role models for fellow pupils.
Principal Lex Hayes said: "Both boys had displayed exceptional talents in their careers here and were popular and valued by both students and staff.
"Aaron and Neil were both prefects and acted as role models for younger pupils. They will be sadly missed by all who knew them in school."
Neil McFerran was a pupil at Monkstown Community School for five years, before changing schools to complete his A-levels.
Andy Conn, one of his teachers, said his death had left the school "stunned".
"He would have been in the school plays, involved in the youth club.
"He would have been a really popular, lively, well thought of boy," Mr Conn said.
The Mayor of Newtownabbey, Alderman Paula Bradley, said she was "devastated" to hear of the teenagers' deaths.
"As a mother of an 18-year-old son, it is all the more hard-hitting," she said.
In the UK, about 50 people die each year in their homes from accidental carbon monoxide poisoning.