Two people are being questioned on suspicion of murder in connection with the deaths of five children in a house fire, police said.
A 38-year-old man from Derby was arrested by detectives investigating the deaths of 10-year-old Jade Philpott, and her siblings John, nine, Jack, seven, Jessie, six, and five-year-old Jayden.
Earlier, Derbyshire Police said a 28-year-old woman was arrested on suspicion of murder. She remains in police custody for questioning.
The children died after a fire broke out at their home in Allenton, Derby, in the early hours. Their 13-year-old brother, Duwayne, who was also in the semi-detached property at the time of the blaze, remains in a critical condition in hospital in Birmingham.
The children's father has been named locally as Mick Philpott, who was dubbed "Shameless Mick" in 2007 for his benefit claims and refusal to get a job in a documentary by former Conservative MP Ann Widdecombe. Mr Philpott, said to be the father of 17 children, hit the headlines after demanding a larger house to share with his wife, girlfriend and eight offspring.
Ms Widdecombe, who spent a week living with the then 57-year-old Philpott for the programme Ann Widdecombe Versus The Benefit Culture, said following the tragedy: "Nobody would ever call him a bad father. I'm so sorry to hear the news and my thoughts are with the family."
Temporary Assistant Chief Constable Steve Cotterill, from Derbyshire Police, said: "There appears to have been valiant attempts by the father to rescue his children."
Mr Cotterill, who said police were keeping an open mind about the cause of the fire, added: "It is a matter of a police inquiry, where we have had to act very quickly to preserve any evidence which may feature later in the inquiry."
Post-mortem examinations will be carried out over the weekend and forensic specialists are examining the house. The six children were asleep in their beds when the fire broke out, and the two adults, including Mr Philpott, were downstairs, police said.
Friends of the family left flowers at the scene. Daniel Walsh, who said he had been a friend of Mr Philpott for many years, called the fire "shocking", adding: "It's absolutely tragic. Any parent's worst nightmare."