Police officers state that the unidentified men fired nine times into the auto carrying the 38-year-old politician and her driver, Anderson Pedro Gomes.
The late Wednesday attack against Marielle Franco, 38, came despite army intervention to control Rio's soaring crime rate, and sparked immediate outrage.
Ms. Franco's driver, Anderson Pedro Gomes, was also killed, when a vehicle pulled up next to theirs at a downtown intersection and someone shot nine bullets into the auto, five of which reportedly hit Ms. Franco in the head.
Raul Jungmann, who heads the federal government's newly created Public Security Ministry, said at an event in Sao Paulo that Franco's killing was "another lamentable, daily tragedy that takes place in Rio de Janeiro".
A press officer was also travelling with Franco and Gomes, but survived the shooting.
Rio's mayor, Marcelo Crivella, declared three days of mourning after Franco's death, and vowed that the perpetrators would face justice.
Franco, who was raised and lived in the Mare complex of slums, long one of Rio's more risky areas, received over 46,500 votes in the 2016 election.
The killing came just weeks after the Brazilian government chose to let the military take over all security operations in Rio to control the city's increasing crime rates.
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Ms. Franco's death comes amid rising criticism of the military intervention in Rio de Janeiro.
Franco's last words was in a tweet bringing attention to police brutality.
In city council elections in 2016 she surprised everyone, including herself, by getting 46,000 votes, the fifth biggest tally of anyone running.
- Ana c!ara otoni (@anaclaraotoni) March 15, 2018#MarielleFranco was an expert on police violence and on Saturday accused officers of being overly aggressive in searching residents of gang-controlled shanty towns.
To be sure, not all Brazilians mourned Franco's death. Mathews Melo was leaving church. She was a resident of one of the most impoverished communities in Rio known as the favela da Maré, and became known for her social work in the country's poorest and violence-impacted communities. "This has always happened and with the (military) intervention things have gotten worse", she wrote on her Twitter. This week two young men were killed ...
Federal police said in a statement that they were investigating "the origin of the ammunition and the circumstances of the casings found at the scene of the crime". Finance Minister Henrique Meirelles, who is contemplating a bid for the presidency, said the assassination underscores the need of the Rio military operation. Franco was a source of pride and hope particularly for Black youth and women, as she fought against the people in power on behalf of the neglected people of the favelas.
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