Four months after the unsolved murder of black Rio de Janeiro councillor and human rights defender Marielle Franco, Amnesty International has called for independent monitoring of the police investigation, saying that the lack of progress in the case demonstrated the “incompetence” of Brazil’s criminal justice system.

Franco and her driver Anderson Gomes were shot dead on 14 March after leaving a public meeting. The crime prompted protests across Brazil, as well as international condemnation and calls for an independent investigation that have so far been ignored.

“After four months, the lack of a resolution in the murder of Marielle Franco shows inefficiency, incompetence and a lack of will in the Brazilian criminal justice institutions,” Jurema Werneck, the executive director of Amnesty International Brazil, said in a statement.

On Wednesday, Pakistani activist and Nobel peace prize laureate Malala Yousafzai helped stencil a graffiti design of Marielle Franco on the wall of a Rio favela. “I know she inspired many Brazilian women and girls and I know they will carry her legacy forward,” Yousafzai said.

Franco had fiercely criticised police killings in the Rio 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. Know as “militias”, these groups often include police officers.

“In the Marielle case there are strong indications of the involvement of state and security agents,” Renata Neder, a research and policy coordinator for Amnesty International Brazil, told reporters, citing local media reports.

Brazilian media have reported that ammunition used in Franco’s killing was restricted calibre taken from a federal police supply, and that six Heckler & Koch MP5 sub-machines – believed to be the murder weapon – have disappeared from police arsenals since 2011.

Security cameras at the murder scene were disconnected on the eve of the crime, media reports have said. A testifying witness told police that a Rio councillor linked to militias and a jailed former police officer had planned the killing – which both men have denied.

Amnesty’s Neder said an independent group should be set up to monitor the investigation along the lines of the four-person group of experts set up by the Inter-American Commission on Human Rights and Organisation of American States to investigate violence in Nicaragua.

“What is important is that this is a group of specialists, jurists, forensic experts who don’t have any conflict of interest with this case in particular and are not part of the state apparatus,” Neder said. “In Rio de Janeiro the police don’t usually investigate homicides that police officers participated in.”

Franco’s parents said they believed police can solve the case but their frustrations are growing over the delay.

“It seems to me we are reaching a point of impunity,” said Marinete da Silva, describing the “deep pain” she feels over the death of her daughter as what would have been her 39^th birthday on 27 July nears.

“Why did they do this to my daughter?” her father, Antonio da Silva, said. “This question won’t leave my mouth – and I don’t have any answer.”

A police spokesman said details of the case had not been released “as a way of protecting witnesses and relatives in order to ensure the success of the homicide investigation”.

Dom Phillips

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