A 38-year-old man has been arrested on suspicion of murder in connection with a house fire in Derby which killed five children, according to Sky sources.
It follows the arrest of a 28-year-old woman on suspicion of murder, who remains in police custody.
The brothers and sisters who died in the blaze were earlier named by police as Jade Philpott, aged 10, nine-year-old John, Jack, aged seven, Jessie, six, and Jayden, who was five.
Their 13-year-old brother, Duwayne, remains in a critical condition in hospital in Birmingham. Police say his parents are by his bedside.
Post-mortems will be carried out during the weekend to determine the exact cause of the children's deaths.
Derbyshire Police Assistant Chief Constable Steve Cotterill announced the arrest at a news conference but stressed that detectives were keeping an open mind about the cause of the blaze.
The woman was arrested "elsewhere in the city", he said.
He added: "It appears at this stage of the investigation that the children were in bed asleep upstairs and the parents were downstairs.
Mr Cotterill said the father had made a "valiant attempt" to save the youngsters at the house in Allenton.
He has been named locally as Mick Philpott.
On April 22, Mr Philpott - said to be the father of 17 children - updated his open access Facebook profile and revealed how much he adored his kids.
He said: "These children are my life, much more than life itself.
Mr Philpott made national newspaper headlines several years ago over his attempts to get a larger council house for his family.
"All I've ever wanted is to do right by my family. My children are the most important thing in my life."
In 2007, former Conservative MP Ann Widdecombe spent a week living with Mr Philpott as part of the ITV programme Ann Widdecombe Versus The Benefit Culture.
After news of the fire, she told the Huffington Post UK: "Nobody would ever call him a bad father. I'm so sorry to hear the news and my thoughts are with the family."
Derbyshire Fire and Rescue Service chief fire officer Sean Frayne said 30 firefighters were sent to the scene.
"When the first crew arrived on scene, they were faced with a fire at the front of the building," Mr Frayne said.
"They quickly made their way past the fire to search the rest of the premises, having been told that persons were inside the house."
A neighbour, who gave her name only as Kaye, said she saw flames coming out of the house and "loads of ambulances", described it as a "horrible, horrible" scene.
"I just saw the babies going to the ambulance because it was parked outside our house," she said.
All the children who died went to St George's Catholic Primary School in Littleover, Derby, a school spokeswoman confirmed.
A statement from the school said: "We extend our deepest sympathies to the family and community at this sad time.
"Whilst this is under police investigation, we are unable to make further comment."
:: An incident room has been set up at St Mary's Wharf police station. Police ask anyone with information to call 101 or Crimestoppers on 0800 555 111 .