Two former police officers have been arrested over the murder of the Rio de Janeiro councillor Marielle Franco, two days before the first anniversary of her death, which prompted international outrage.

Franco, a groundbreaking politician who was born in one of Rio’s largest favelas and became a voice for disadvantaged people in the city, was killed in a drive-by shooting along with her driver, Anderson Gomes.

She had criticised police killings in the favelas where she grew up and took part in a 2008 state legislature inquiry into the paramilitary gangs that dominate large areas of Rio state. Known as militias, these groups often include police officers.

The suspects were identified as Ronnie Lessa, a retired military police officer, and Élcio Vieira de Queiroz, a former police officer.

Prosecutors from a special organised crime unit said in a statement that the shooting was meticulously planned over the course of three months. “It is incontestable that Marielle Franco was summarily executed for her political activity in the defence of the causes she defended,” they added.