A 21-year-old mother was being held on suspicion of murdering her baby and toddler today after they were found stabbed to death.
The bodies of brothers Romario and Delanyo Mullings-Sewell, aged two and just three months, were found when police stormed their home.
Police had visited the house in Manchester hours earlier after worried neighbours reported seeing a woman 'running around' screaming she was going to be shot.
When they found no one home, they searched the area and left. Just three hours later, residents raised the alarm again after hearing children screaming.
By the time officers arrived for the second time, both boys were already dead. They each had a single stab wound to the stomach.
Their mother, Jael Mullings, is being held on suspicion of murder and has been sectioned under the Mental Health Act.
Greater Manchester Police's response to the initial calls is now being investigated by the forces' watchdog, the Independent Police Complaints Commission.
Alerted by the local doctor's surgery, officers went to the house in the Cheetham Hill area shortly before 3pm and knocked on the doors and windows but no one was in.
A neighbour saw them and said she had seen the mother pushing a buggy nearby so they searched the area.
Finding nothing, they went to the children's grandmother's house to try to track down Ms Mullings who was described as being in a 'distressed state'.
While they were there, paramedics were called to the family home and the children found dead at which point the police were contacted again.
An IPCC spokesman said there were two different accounts from police about contact made with the family before the deaths.
He said: 'We know there was some form of contact by the police. We need to determine what the police did in response to this contact.'
Explaining the apparently 'correct' version of events, he added: 'Police were contacted with "expressions of concern" about the welfare of the woman.
'They went round to the property at about 2.50pm yesterday. They searched the area because a neighbour said she saw the woman walking away with her children. The next call they got was that the children were dead.
'We'll be asking what the police were told, what the concerns were, what the officers did, and decide whether there's anything there to take forward into an investigation.'
A police spokesman confirmed officers had visited the house at 2.50pm because of reports of 'concern for welfare' but had not gone inside.
He said that it was while officers were at the grandparents' house that a 999  call was made suggesting the children were dead.
'A 21-year-old woman has been arrested on suspicion of murder and is currently in police custody for questioning,' the spokesman said. 'An investigation is under way into the circumstances surrounding their deaths.'
The boys' distraught family paid tribute to the two 'beautiful, innocent children' today.
In a statement, they said: 'We, as a family, are struggling to come to terms with the tragic events of yesterday afternoon and we cannot even begin to understand what happened.
'Everybody is completely devastated... We hope that wherever the boys have gone to, they are at peace.'
Melissa Bell, who lived near the family in the Cheetham Hill area of Manchester, added: 'They were well-behaved kids, not rowdy. They were gorgeous. Every time I saw her she was with the kids.
'They were just gorgeous, beautiful, the three-month-old had just started to get a personality of his own.'
She claimed the killings must have been a 'cry for help' by their mother: 'She never got the help she needed. She is a lovely girl. Everyone said she needed help but she never got any.
'Everyone gets down when you have a kid. It's got to be depression for her to do something so desperate like that. Everyone has bad days with a baby.'
Other neighbours also said the young mother had been troubled in recent months and described how she was seen shouting to herself in the street before her sons died.
One woman, who did not wish to be named told how a woman had appeared to go 'beserk' earlier in the afternoon.
'She was running around screaming,' she said. 'It was about midday when she was screaming and then later on in the afternoon the kids from the area were playing in the street and she ran over to them and shouted "stop f***ing laughing at my family. They were only little kids".'
The road is cordoned off as investigations continue into the children's deaths
The house was sealed off today and being guarded by police as forensic experts continued their work. Post mortems on the two boys are expected to be carried out later today.
At the crime scene, Det. Sup. Shaun Donnellan, said: 'This is a tragedy, there are two children dead, they appear to be the victims of a fatal attack and the mother in custody who is being detained under the Mental Health Act.
'It's too early to speculate, it's too raw to the family.'
He said the matter had been referred to the IPCC as a matter of course because of the earlier police involvement.
Police were initially alerted because there appeared to be 'an unpleasant situation arising' and because of the woman's behaviour, the detective explained.
Both the children's father and the officers who found the children's bodies were being counselled for shock.
'It's impossible to try to understand what they were faced with other than to say it's something no human being should ever have to see in their life,' he said.He also revealed that Miss Mullings was known to police over minor criminal matters unrelated to the events yesterday.