text: An 81-year-old man has been arrested for allegedly stabbing to death his granddaughter's ex-boyfriend after the young couple started arguing following their daughter's first birthday party. Nicolas Diaz reportedly intervened after his granddaughter, Amber Diaz, and her former partner, Kevin Rivera, 21, got into a dispute at their home in Brooklyn, New York, in the early hours of Sunday. The recently separated couple, who have a baby daughter, Aeralye, together, had just returned to the apartment after celebrating the infant's first birthday. Diaz wanted to break up the fight, relatives said. He allegedly stabbed Mr Rivera, an unemployed musician, with a knife, before fleeing the scene. When medics arrived at the Bushwick property, they discovered Mr Rivera with a chest wound. Scroll down for video . 'Manslaughter': Nicolas Diaz (Ieft), 81, allegedly stabbed to death his granddaughter's ex-boyfriend, Kevin Rivera, (right), after the couple started arguing following their daughter's first birthday party on Sunday . Scene: Diaz reportedly intervened after his granddaughter, Amber Diaz, and 21-year-old Mr Rivera got into a dispute at their home in this apartment bock at 1225 Broadway (pictured) in Bushwick in Brooklyn, New York . He was rushed to Woodhull Medical Center, but despite medics' best efforts, was later pronounced dead, according to the New York Times. Police immediately launched a manhunt for Diaz. On Monday morning, the suspect turned himself into police, NBC reported. He has since been charged with manslaughter and criminal possession of a weapon in relation to the killing. Miss Diaz and Mr Rivera had apparently been dating for around three years before they split. Relatives told the Times their relationship had started to break down following Aeralye's birth last year. 'They were always fighting. Girlfriend and boyfriend stuff,' said Jose Rivera, the victim’s grandfather, adding he had adopted his grandson, along with his twin brother and sister, after their father had died. Police: The pensioner allegedly stabbed Mr Rivera with a knife, before fleeing the scene. When medics arrived at the property, they found Mr Rivera with a chest wound. Above, a cop stands at the entrance to the home . Grief-stricken: Mr Rivera was rushed to Woodhull Medical Center, but despite medics' best efforts, was later pronounced dead. Above, some of the victim's devastated relatives are pictured hugging on the street . On the day of the alleged manslaughter, the couple had just returned home after celebrating Aeralye's first birthday together. They were arguing when Diaz reportedly lashed out at Mr Rivera with the knife. 'The girl was screaming,' one neighbor told the Times. 'It was getting out of control. I heard screaming, footsteps - hard.' The neighbor said they then called 911. Police arrived at the scene 20 minutes later. Speaking to the New York Post, Miss Diaz said of her now-deceased ex-boyfriend: 'Kevin is my ex-boyfriend and he was trying to get back with me. He wouldn’t leave, and he was yelling and pushing.' Although Mr Rivera, who also suffered slashes to his arms in the attack, was unemployed at the time of his death, he had apparently considered applying to be a driver for the online taxi service, Uber. Aerlaye has reportedly been taken into custody by officials. Emergency: Miss Diaz and Mr Rivera had apparently been dating for around three years before they split. Relatives told the Times their relationship had started to break down following Aeralye's birth last year .
target: Kevin Rivera and Amber Diaz started arguing after their daughter's party .
  Miss Diaz's grandfather, Nicolas Diaz, 81, allegedly intervened with knife .
  He 'stabbed Mr Rivera in the chest'; fled the scene after Sunday's killing .
  Victim rushed to hospital with stab wound, where he was declared dead .
  Diaz turned himself in to police in Brooklyn, New York, Monday morning .
  Has been charged with manslaughter, while baby girl is now in custody .