Members of the public are being urged to come forward with any information about a flat fire in Langley Mill in which three people died.
Amy Smith, 17, and her six-and-a-half-month-old daughter Ruby-Grace Gaunt, who lived in the flat, died along with friend Edward Green, 17, of Belper, after the blaze took hold in the early hours of Sunday.
Superintendent Jim Allen, of Derbyshire police, said: “We are working closely with the fire service to determine how the blaze started. At this time, we are treating it as unexplained and we are meticulously working through all lines of inquiry to establish exactly what happened that morning.
“Specially trained police officers are working with all the families involved at this distressing time.”
Anyone with information is asked to contact police on 101.