Wednesday's killing had the hallmarks of an assassination. Police say her car may have been tailed several miles before the killer fired, killing Franco and her driver, leaving an aide wounded, then disappearing.
No suspects have been announced by police, but Franco, who was 38, had ruffled many feathers -- and that was her mission.
Black, lesbian and unusually vocal in castigating alleged killings by out-of-control police units in Rio's impoverished favelas, she had no interest in keeping a low profile.
In city council elections in 2016 she surprised everyone, including herself, by getting 46,000 votes, the fifth biggest tally of anyone running. The victory thrust her into a world dominated by white men.
"I was very happy because it was a reply, through the polls, to the people who want to keep us black women from the favelas away from the debates," she said at the time.
Stray bullet
Born in a favela and active in black rights movements, it was not surprising that her last act before getting in the fateful car had been to participate in a debate on black empowerment in the trendy Lapa neighborhood.
"Marielle was smiling, strong, sure of herself and kept her feet on the ground," said Marcela Lisboa, a former fellow activist in the leftist Socialism and Liberty Party (PSOL). "She looked at other people in the eyes and knew she was different to the other politicians."
Franco's background in the huge Mare favela, one of the most violent places in Rio, informed her approach. For years, the Mare has been wracked by wars between narco-trafficking gangs and often heavy handed interventions by police.
Franco says her political life began when she had just signed up for university on a special program for youths from favelas when her friend, also a student, was hit by a stray bullet.
Although she got pregnant at 18, she continued to take night classes and eventually got a scholarship to study at the prestigious PUC college, studying sociology there and later public administration.
In 2006 she became an assistant to Rio state deputy Marcelo Freixo, from the PSOL, who headed a trailblazing committee looking into the activity of shadowy -- and much feared -- militia groups that run mafia-like rackets in favelas.
jpegMpeg4-1280x720When she was elected to the city council, Franco stood out for a bill that would force the city to track statistics on violence against women.
She had also sharply criticized President Michel Temer's decision to put the army in charge of Rio's failing security services. Two weeks ago, she was name to coordinate a committee meant to look out for military abuses.
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