Police have been searching a flat in Long Eaton as part of their investigation into a blaze which killed three people.
Officers and forensic teams spent several hours at the property in New Tythe Street on Tuesday.
Police revealed they were there in relation to the fire on North Street, Langley Mill, at 4.04am on Sunday.
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.
Police have arrested a fourth man on suspicion of murder after the fire.
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.
Anyone with information should call police on 101.