The first murder of the year happened just a few hours into 2020, when Isaiah Usen-Satchell was stabbed to death close to his home in Norfolk Park.
Two men have been charged over his death.
Just a week later, two-year-old Keigan O’Brien died in Doncaster after being admitted to hospital in cardiac arrest.
A post mortem examination revealed the toddler had suffered head injuries.
His mum Sarah O’Brien, 32, and her boyfriend Martin Currie, 36, have been charged with murder.
The most recent death to trigger a murder probe was that of 39-year-old Leighton Owen, who was found dead in a flat in Hatfield House, Grove Place, Balby, Doncaster, shortly after 11pm on Wednesday, August 26.
Two women, aged 45 and 40, and a 21-year-old man, were arrested on suspicion of murder and later released under investigation as enquiries continue.
Anyone with information about any of the deaths should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.
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1. 1) Stabbing
Isaiah Usen-Satchell, aged 18, was stabbed to death on New Years Day in the Norfolk Park area of Sheffield. Twin brothers Jacob Mwanza and Isaac Mwanza,19 and of Rother View Road, Canklow, Rotherham, have been charged. Photo: Claire Lewis
2. 2) Head injuries
Keigan O'Brien, two, died in hospital on January 9 after being admitted in cardiac arrest. A post mortem revealed he had suffered head injuries. His mum Sarah O’Brien, aged 32, and her boyfriend Martin Currie, 36, both of Bosworth Road, Adwick, Doncaster, have been charged. Photo: Claire Lewis
3. 3) Stabbing
Jordan Marples-Douglas, 23, was stabbed to death in his home in Woodthorpe , Sheffield, on March 6. Two men - Ben Jones, 25, of no fixed address and Aaron Johnson, 21, of Mawfa Crescent, Norton - have been charged with murder. Photo: Claire Lewis
4. 4) Assault
Xiangyu Li, 26, died after an attack in the Union Street area of Sheffield city centre on March 24. He was visiting friends in the city from his home in Kent at the time. Yongqi Liang, 25, of Broomhall Street, Broomhall, has been charged with murder. Photo: Claire Lewis