- Video report by ITV News Correspondent Juliet Bremner
A sixth person has been arrested in connection with a "targeted", suspected arson attack that killed three siblings, and left a fourth child and their mother fighting for their lives.
A 25-year-old man was arrested on suspicion of murder on Tuesday, a day after three men, aged 19, 20 and 23 and a 20-year-old woman were arrested on suspicion of the same offence following the fatal fire in Walkden, Greater Manchester.
A fourth man, aged 24, has been arrested on suspicion of assisting an offender.
Speaking on Tuesday afternoon, Greater Manchester Police said that after reviewing CCTV they believe the fire was a "targeted attack on the house".
It is understood one line of inquiry is that the culprit may have climbed up scaffolding outside the property to pour fuel down the chimney.
Fire investigators have used an aerial platform to examine the location.
Firefighters were called to the mid-terraced house in Jackson Street, just before 5am on Monday.
A 14-year-old girl, named locally as Demi Pearson, was declared dead at the scene.
Eight-year-old Brandon Pearson and seven-year-old Lacie Pearson, were taken to hospital but later died.
The children's 35-year-old mother, Michelle Pearson, is in a serious condition in hospital while a fourth sibling, a three-year-old girl, believed to be called Lia, is in a critical condition.
Ms Pearson remains heavily sedated in hospital and is not aware that her children have died.
While none of the victims have been formally identified, they have been named by police.
The 19-year-old and 20-year-old who were arrested on suspicion of murder, and the 24-year-old who was arrested on suspicion of assisting an offender have all been released on police bail.
The others remain in police custody for questioning.
Chief Superintendent Wayne Miller of Greater Manchester Police said: "I want to describe what this incident is, it is the murder using fire of three children and we have a three-year-old girl fighting for her life who is seriously ill."
In a separate news conference on Tuesday afternoon, he added: "I cannot imagine what their family are going through.
"They are obviously completely devastated, the loss of a child in any circumstances is unthinkable, but to lose three in such deplorable circumstances, words cannot describe.
"My heart breaks for them, it really does."
Older brother Kyle Pearson, 16, managed to escape the blaze with his friend Bobby Harris through an upstairs window before fire crews arrived.
He told the Manchester Evening News: "I fell asleep and the next thing I knew was I could hear my mum screaming, 'Fire!' There was lots of smoke so I climbed out of an upstairs window.
"I’ve gone to get back in but a cloud of smoke hit me in the face.
"I tried to break the front door down and smashed a window, but I couldn’t get in because of the flames and the smoke.
"I can’t get my head around it. It’s disgraceful."
Demi was a pupil at Harrop Fold School in Salford, which featured in the Channel 4 documentary Educating Greater Manchester.
Head teacher Drew Povey said: "We are devastated at the tragic loss of life today in our community. Our thoughts and prayers are with the family involved."
Police had been called to the property three hours before the fire with reports that their bin had been set alight.
Greater Manchester Police confirmed there were "earlier incidents" at the address and said the force has made a self-referral to the Independent Police Complaints Commission (IPCC).
IPCC acting deputy director of operations for the North of England Steve Noonan said: "This is a most tragic incident in which three young lives have been lost and our thoughts and sympathies are with the family and friends of the children who sadly died and those injured.
"We have carefully assessed the referral from Greater Manchester Police and, as there was prior police contact, feel that it is necessary to independently investigate the circumstances of this incident in relation to the force's actions."
A large police presence remained at the scene throughout the day and both ends of the street were cordoned off.
Tearful locals stood at the end of the cordons and consoled each other with embraces.
One woman, who did not wish to be named, said she knew the mother of the family, Michelle Pearson.
She said: "I have known Michelle for years. The family have lived here for a long time. We are all just in shock.
"There has been trouble round here before but it's more wannabe gangsters, teenage stuff."
Police praised the public for coming forward with information which has helped them "make progress" in the investigation, but urged anyone else with information to come forward.
- Anyone with any information should contact police on 0161 856 8797, or call 101 or the independent charity Crimestoppers, anonymously, on 0800 555111.