A 17-year-old youth has been arrested on suspicion of attempted murder after three members of his family suffered serious stab wounds.
West Midlands Police said no one else was being sought in connection with the "isolated, self-contained and tragic" incident at an end-of-terrace house in Small Heath, Birmingham.
The victims, a woman aged in her 40s and two sisters aged 16 and 13, are all believed to have suffered " significant" multiple knife injuries at about 7.20am today.
Officers were called to the property in Little Green Lane by a resident who reported hearing screams.
A police spokesman said: "Officers arrived at the scene within a matter of minutes and arrested a man in the street and recovered a knife nearby.
"He has been taken to hospital having suffered a minor cut to his hand."
A baby was discovered unharmed inside the address and is currently being cared for by family members.
Confirming that the incident was in no way linked to the racist murder of pensioner Mohammed Saleem in the same street last April, Chief Inspector Dean Hatton said the youth under arrest was believed to be the brother of two of the victims.
The officer said: "We don't believe there are any issues for the wider community. We believe this is an isolated tragic incident involving one family."
Neighbours said the family who lived at the address had moved to Britain from the Middle East but were originally from Pakistan.
One man, who declined to be named, said: "They are a very good family and what has happened is terrible.
"They are a very loving family, I saw them shopping together and it's impossible to imagine why this has happened."
An air ambulance was called to the scene, where other witnesses claimed to have seen the youngest victim lying between two stationary cars.
One man, who also asked not to be named, said: "I didn't see what happened but it must have been terrifying.
"I would describe them as a perfectly ordinary family and we obviously all hope they're OK."
The father of the teenagers, said by neighbours to be working away in Saudi Arabia, is understood to have been made aware of the incident by police.
The youth was arrested near the address and is reported to have gone willingly with police after officers arrived at the scene.
Another local resident described how he saw blood on the baby's neck as the child was being held by her mother, although police have confirmed the infant was unhurt.
Describing how the incident spilled out on to the street, the man, who gave his name only as Geoff, told BBC WM: "I just heard the lady screaming, I got dressed and went to go and see what was happening.
"I saw the lady with a little baby on her lap and my friend said 'watch it, he's over there'. I then carried on looking at him until the police arrived and they arrested him."