RIO DE JANEIRO — The killing of the Greek ambassador was shocking even by the standards of this crime-weary city: Investigators say his wife had him murdered in a home they owned in Rio by her lover, a police officer, who then set the diplomat’s remains on fire.

Police investigators on Friday arrested Françoise Oliveira, the Brazilian wife of the Greek envoy, Kyriakos Amiridis, and Sérgio Gomes Moreira, the officer who confessed to killing the ambassador while having an affair with Ms. Oliveira.

“This was a tragic, cowardly act,” said Evaristo Magalhães, the lead investigator in the case. He said that Ms. Oliveira, 40, and Mr. Moreira, 29, plotted the killing on Sunday before the officer carried it out on Monday.

Ms. Oliveira tried to mislead investigators by saying her husband, 59, had disappeared, Mr. Magalhães said, insisting that she was innocent before confessing on Friday that her lover had killed him. In tears, Ms. Oliveira said that the ambassador’s death “could not be avoided,” Mr. Magalhães said.

The case provided a gruesome finish to the year, as Rio reels from a harrowing crime wave, a financial crisis and graft scandals, just months after hosting the Olympic Games.

The suspected murder of the ambassador by an officer also trains scrutiny on Rio’s police forces, already under pressure over extrajudicial killings, torture, forming militias and assassinating candidates in municipal elections.

Ms. Oliveira is connected to another violent episode in Rio, the 2003 killing of Todd Staheli, an American executive for the oil giant Shell, and his wife, Michelle. The handyman who confessed to the murders was arrested after he allegedly jumped the wall into the nearby home where Ms. Oliveira lived with Mr. Amiridis, who was Greece’s consul in Rio at the time.

“Until then, no one knew that Amiridis was married,” said Hildegard Angel, a columnist who writes about Rio’s high-society and diplomatic circles, describing the Greek diplomat as “outgoing and extremely charming.”

The police got a break in the killing of Mr. Amiridis when a man identified as a cousin of Mr. Moreira told investigators that Ms. Oliveira had offered him about $25,000 to assist in the murder. The cousin, Eduardo Moreira de Melo, 24, was also arrested on Friday.