Detectives investigating the fatal flat fire at Langley Mill have arrested a fourth man on suspicion of murder.
The 24-year-old, of Long Eaton, was arrested on Wednesday morning. Three other men, who were arrested on Monday night, remain in custody.
Officers have obtained a warrant of further detention from magistrates to continue questioning the 17-year-old, who is from Long Eaton, and 21 and 43-year-old, both from Sandiacre.
Police were called to flats in North Street at 4.04am on Sunday following reports of a fire.
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, who is from Belper.
Two men, aged 18 and 17, who were also inside the premises at the time, were rescued by neighbours and treated for smoke inhalation.
Anyone with information should call police on 101.