Former military police officer Alan Morais Nogueira was detained on July 24 on suspicion of having been in the car that fired on the vehicle containing Black activist and Rio de Janeiro city councillor Marielle Franco, killing her and her driver Anderson Gomes on March 14.
Former fire fighter Luis Claudio Ferreira Barbosa, accused of being in the assassin's car, was also taken into custody.
Both men are suspected of being part of a militia group controlled by Orlando Oliveira de Araujo, also known as Orlando da Curicica, and of killing two military police officer one current and one former, at the behest of the illegal organisation.
Orlando Curicia is a former police officer currently serving out a jail term in Rio de Janeiro's state penitentiary, Bangu 9.
An anonymous informant held at the Homicide Precinct of Rio de Janeiro provided authorities with a tip off that led to the arrest of the two men. The informant noted that Morais Nogueira once worked in the Mare barracks in the favela (shantytown) community of Complexo da Mare, where Marielle was raised.
The informant also revealed that two other men were accompanying the former police officer and former fire fighter in the silver Cobalt that fired on Marielle's vehicle.
Investigators believe that Morais Nogueira can help reconstruct the events that claimed the life of Marielle and her driver on March 14 and clarify the motives behind the killings, according to O Povo.
In May, media reports indicated that an unidentified witness had given testimony to federal police claiming that Orlando Curicia and Rio de Janeiro city councillor聽Marcello Siciliano were responsible for ordering Marielle's execution.
According to the witness, who claims to have worked for a militia led by聽Curicicia聽in the city's west, the motive for the assassination was聽Marielle's community activism in areas controlled by the group.
The witness stated that the order to execute Marielle came directly from聽Curicica, who used a mobile phone inside the prison to contact his associates and give the order.
Marielle and Gomes were聽executed in a barrage of bullets fired at her car聽while returning home from an event in central Rio de Janeiro called "Young Black Women Moving Structures".
Three days before she was murdered, Marielle denounced the deaths of two youths during a military police operation in the Acari favela.
鈥淲e must speak loudly so that everybody knows what is happening in Acari right now. The聽41st Military Police Battalion聽of Rio de Janeiro is terrorising and violating聽Acari聽residents.
鈥淭his week two youths were killed and tossed in a ditch. Today, the police walked the streets threatening residents.
鈥淭his has always happened, and with the military intervention things have gotten worse,鈥 she wrote on Twitter.
Two weeks earlier Franco had also been named a rapporteur in the special commission established by the city council to monitor the military intervention in Rio de Janeiro.
Investigators have revealed that the 9mm bullets that killed Marielle were part of a lot bought by federal police in 2006.
[Abridged from .]