Three children found dead at a suburban semi-detached house in Bristol were a seven-year-old boy, a three-year-old girl and a 10-month-old boy, police said on Monday.

Avon and Somerset police said a 42-year-old woman arrested on suspicion of their murder had sustained non-lethal injuries.

Police said it could take more than a week to establish the cause of the children’s deaths and described the incident as highly sensitive and complex. Friends of the family have said there was a “family problem” at the address earlier this year.

Police have refused to clarify the relationship between the children and the woman, but said it was an “isolated” incident and that they did not believe there was a risk to the wider community.

Speaking at the scene in Sea Mills, Ch Insp Vicks Hayward-Melen, said: “This is a terrible and deeply distressing tragedy in which three young children have lost their lives. Our thoughts are very much with the children’s loved ones who are going through a horrendous ordeal.”

Police were called by a member of the public concerned for the welfare of the family in the early hours of Sunday, said Hayward-Melen. “Officers arrived at 12.40am and found three children whose lives could not be saved.” Postmortems were not likely to be completed until the middle or end of the week.

Hayward-Melen said: “I know people will be eager to have answers, but the major crime investigation team are in the very early stages of what will be a highly sensitive, complex and thorough investigation and it will take time to establish all the facts.”

The Independent Office for Police Conduct (IOPC) said the force had informed it about the incident because of previous police contact in February.

Hayward-Melen said: “As is usual in circumstances where there’s been prior police contact, we have notified the [IOPC], and a mandatory formal referral will be made today.”

She added: “We’ve been humbled by the community response to this tragedy. At a time of great sadness and disbelief, we’ve seen spontaneous acts of care and support. Within hours of the incident occurring, a local church was opened up for people to gather and mourn. This is what being in a community is all about and we’d like to thank all those who continue to offer this important and selfless service.”

The street where the incident took place is in a quiet neighbourhood about four miles north-west of Bristol city centre. Police officers remained posted outside a semi-detached house in the street on Monday morning.

A woman, who did not wish to be named, described the arrested woman as “lovely”. She said: “She was so happy when she had that little boy. We were so happy for her. We came round and gave gifts. She was really gentle, really lovely. She always had a smile on her face.” The neighbour said the woman was of Sudanese heritage.

A Methodist church hall was opened by police on Sunday evening to give members of the Sudanese community a place to meet and comfort each another.

A Bristol minicab driver who knows the family well said he had last seen the mother and children about a fortnight ago.

“I visited them because I heard there was a family problem between [the children’s mother and father]. I wanted to find out if we could help,” he said.

He said the father, who worked as a security guard, was not living at the house at that point. “She wasn’t happy with her husband,” he added. “Her kids were all happy. I asked them whether they needed my help as a taxi driver. They asked me to take them to Tesco. They did their shopping then I took them back home.

“She is a very nice person. It is very sad. Sudanese are normally patient people. You would not have these sort of issues normally. We have a very close community here. I heard about what had happened and came over.” The minicab driver said the eldest child had been born in Sudan and the others were born in the UK.

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