Investigators said Kyriakos Amiridis is thought to have been stabbed to death by 29-year-old police officer Sergio Moreira at the behest of the diplomat’s Brazilian wife, Francoise Amiridis.
According to Moreira, he and Mrs Amiridis, 40, were romantically involved and planned the killing together.
Yet she has denied any involvement in her husband’s death.
One investigator said: ‘She falls into contradictions. She started crying a lot and said it was the police officer who was the author of her husband’s murder.’
Mr Amiridis is believed to have been stabbed to death at the couple’s home after blood was found on a sofa.
Investigators say Moreira wanted to burn the ambassador’s body to cover his tracks.
‘Moreira says he got into a physical fight with the ambassador and he had no choice other than to kill him,’ the investigator added.
‘He says he was desperate and didn’t know what to do so he asked a cousin for help.’
Police have since detained the cousin, who they say acted as a lookout during the attack and helped carry the body from the house with the promise that he would be paid 80,000 reais (£20,000).
Mr Amiridis served as Greece’s consul general in Rio from 2001 to 2004, before taking the country’s top diplomatic post in 2016.
His wife and the officer remain in police custody, but have not been charged.