The charismatic, black, lesbian city councilor was a vocal critic of police brutality and corruption, and her murder in a drive-by-shooting stunned Brazil.

The federal police's nearly 500-page investigation report is packed with astonishing revelations about a decades-old problem in a city plagued by violence.

It recounts in minute detail the functioning of the militia squads that sow terror in Rio's working-class neighborhoods, with the complicity of police officers and high-ranking political figures.

The militias -- which gained notoriety in Brazil's popular "Elite Squad" movies -- first formed around four decades ago when former police officers and soldiers banded together to offer protection from violent drug cartels.

They then transformed into powerful criminal organizations that control large swathes of the city.

Franco often spoke out against the militias, before she was gunned down in her car, at the age of 38, along with her driver, in 2018.

Former leftist lawmaker Marcelo Freixo, who was a mentor to Franco, said the investigation was "fundamental... to understand the depth of the abyss in which Rio finds itself."

The probe "shows that the militias retain a strong influence in the highest echelons of the police and that political power plays an active role in favoring their influence", Carolina Grillo, a sociologist with the Fluminense Federal University (UFF), told AFP.

- 'Ensure impunity' -

Two of the alleged masterminds of Franco's murder -- the brothers Domingos and Chiquinho Brazao -- are veterans of Rio politics whom the investigation links directly to the militia.

Chiquinho had been a city councilor and is currently a national congressman while Domingos served as a municipal congressman and is currently an advisor with the state auditor.

The third suspect arrested on Sunday was none other than the former head of Rio's civil police, Rivaldo Barbosa, who was initially in charge of the investigation.

All three men proclaim their innocence.

Two former police officers -- the gunman and getaway driver -- were arrested a year after the crime.

Investigators say the Brazao brothers roped Barbosa into their scheme to "ensure impunity beforehand."

Barbosa, who was appointed to the role of civil police chief the day before the murder, was to ensure "that the investigation was stillborn" by covering the killers' tracks.

It was he who spoke to and comforted Franco's family in the wake of her death, and his alleged involvement came as a shock to her loved ones.

"He told me that it was a matter of honor for him to solve this crime," Franco's mother, Marinete da Silva, told the Globo news channel.

The investigation dragged on for five years, before being taken over by the federal police after President Luiz Inacio Lula da Silva returned to office in January 2023.

- Land grabbing -

Beyond the rampant extortion of residents, the militias have seized public land to illegally build housing or commercial buildings.

Grillo explains that real estate has become their "main source of income."

The investigation report also mentions "numerous clues" to their involvement "in the criminal activities of militias linked to the illegal appropriation of land."

Franco fought hard against land-grabbing in communities controlled by the militia.

The federal police believe that the Brazao brothers had Franco killed because she "threatened their interests."

This investigation "showed the key importance of the land question" in the influence that the militias have in neighborhoods under their control, including "on the political level," said David Marques, of the NGO Brazilian Public Security Forum.

Areas dominated by these criminal groups have become electoral strongholds for politicians like the Brazao brothers.

In 2008, the lawmaker Marcelo Freixo established a parliamentary commission of inquiry in Rio's Legislative Assembly that dealt a heavy blow to the militias.

Dozens of people were arrested, including local elected officials.

But these criminal groups "quickly reformed and resumed their growth," said Grillo.

Human Rights Minister Silvio Almeida said that the turmoil unfurled by the revelations within the Franco investigation presented an opportunity to take back control of territory controlled by the militias.

"To do this, we need public policies, because it is by taking advantage of this vacuum that the militias move in and sow terror," he said Tuesday during a tribute to Franco.