An explosion which killed a man and three children at a house in Staffordshire was caused deliberately, police have said.
The children perished in the fire along with their mother's partner
Roderick Hine, 44, died along with Samantha Carter, 16, Patricia Carter, 10, and Marcus Carter, 7, at the house in Cheddleton, Staffs on Thursday.
The children's family said they were "shocked and totally devastated".
A murder inquiry is now under way and police have more time to question a 31-year-old man they have in custody.
Police say he is not the father of the three children.
Mr Hine, his partner Amanda Carter and her children were described by relatives as "a close, loving family" with Mr Hine and Marcus particularly close, sharing a love of motorbikes.
In a statement, the children's family added: "Tricia was a beautiful little girl who loved her dresses".
Samantha, who would have been 17 in May, had just started working in a shop in the Leek area of Staffordshire and was looking forward to learning to drive with a cousin, they added.
The children's mother, Amanda, was at work when the incident happened at the three-bedroom house.
Post-mortem examinations are being carried out on Saturday and work is continuing on stabilising the building which is unsafe.
Mr Hine shared his love of motorbikes with Marcus Carter
Det Ch Insp Dave Garrett said: "The scene is one of the most challenging ever encountered by Staffordshire Police.
"We have to ensure any evidence inside the building and from the surrounding scene is protected and gathered. This is a painstaking task which could take several days."
The blast, in Hillside Road, developed into a severe fire.
Six houses surrounding the home were evacuated because of the blaze on Thursday night and part of the road remained sealed off late on Friday.
CCTV pictures recorded from a neighbour's garden, obtained by the BBC, show the moment the explosion happened, followed by at least two other blasts.
Eye-witness Audrey Spooner said: "It was just an inferno, I've never seen anything like it."
Roy Spooner said: "There were flames coming out of the roof and there was fire balls coming out from above the windows."
Staffordshire fire service spokesman Neil Backes said: "It's a tragic situation. Our officers tend to rely heavily on their experience during these times.
"It's not something we're unaccustomed to, but it never ceases to surprise and shock us when these things happen."