ITV News Correspondent Ben Chapman reports on the tragedy that has shocked a small town on the outskirts of Sheffield
The grandparents of two children found dead in a house along with their mother and another child said they are "heartbroken" as they struggled to hold back tears at the site of the tragedy.
John-Paul Bennett, 13, and Lacey Bennett, 11 were found alongside their mother, Terri Harris, 35 and Lacey's friend Connie Gent, 11, from Sheffield at the house in Killamarsh on Sunday.
"We're just devastated", they said, stood outside the house where the bodies were found.
"They were the most well-mannered children you would ever meet.
"They always remembered their manners, always please and thank you."
Father, Jason Bennett was also seen at the scene where he stayed for more than half-an-hour, clearly distraught and being comforted by a friend or relative.
Posting on Facebook he said: "Life will never be the same. My angels."
"Absolutely broken my heart into a million pieces."
Chief Constable Rachel Swann confirmed they had arrested a 31-year-old man from Killamarsh and were not looking for anyone else.
She said: "This has been an understandably shocking event that has deeply affected the local community here in Killamarsh, as well as Connie's family and friends in Sheffield and our thoughts are also with them."
"While understandably worrying and upsetting this is believed to be an isolated incident, in which those involved were known to each other and nobody else is being sought," she added.
Around 300 people, including many children, gathered for a vigil in a park a few hundred metres from the scene in Chandos Crescent, Killamarsh, on Monday evening.
Many brought flowers, teddy bears, balloons reading "RIP" and lit candles, while some were in tears as the pastor of Killamarsh Church on the Street, David Taylor, led them in a prayer.
Mr Taylor said: “What do you say at time like this? It’s so difficult.”
The pastor said: “I can’t give you any answers. I don’t have the answers. But each one of you is here to remember four people who are no longer with us.
“And it is a day that we’ll all remember because this doesn’t happen in Killamarsh. But it has".
Derbyshire Police said they were called to the house at around 7.25am on Sunday and a man remains in custody.
One man living close by said: "I'm shocked. I think people will be devastated."
The man, who has lived in his home for just over a year, said he believed the family had been there for five or six months.
He said he was alerted by police on Sunday morning and thought: "God, what's happened here?"
Chandos Crescent remains sealed off with a number of police officers patrolling the street and many police vehicles parked in the surrounding streets.
A Sheffield secondary school announced it would close after three of its students were involved in an incident and has postponed an open evening on Tuesday.
Writing on Twitter, Outwood City Academy said with "great sadness" the school would not be open as usual for teaching.
Police have urged anyone with information about the incident to come forward.
Detective Chief Inspector Rob Routledge, who is leading the investigation, said on Sunday evening: "We would really like to hear from anyone who saw or heard anything suspicious in the early hours of this morning, or has any information which they think could help with our inquiries.
"It is understandably a worrying time for the community...it was important to us that close relatives of those involved were informed before we could give further details about incident."
North East Derbyshire MP Lee Rowley offered his condolences following the "terrible and hugely tragic" incident.
"Terrible and hugely tragic news from Killamarsh this evening. Our condolences to the family and our thoughts are with them," Mr Rowley said on Twitter.