An Argentine legislator shot dead in front of the congress building in the capital city of Buenos Aires last Thursday died of his wounds.
The police believe the shooting was not politically motivated and that the target was a friend of Miguel Marcelo Yadón of Olivares.
The two men were shot at from a car. Six people were arrested.
Mr. Yadón, 58, was hit five times and died on the spot.
Olivares was hit by three bullets and died in hospital on Sunday.
"Yadón was the murder target and our hypothesis is that it was a personal attack," said Patricia Bullrich, Minister of Public Security.
Local media reported that all detainees allegedly belonged to the same criminal gang.
They also said that the motive for the killing could have been an alleged relationship that Mr. Yadón had with the daughter of one of the suspects.
Argentine President Mauricio Macri mourned Mr Olivares, whom he calls a hard-working, popular and upstanding man, for two days.