Mr. Lessa, whom federal prosecutors have identified as the man responsible for carrying out the assassination, is a retired military police sergeant and was decorated by the Rio de Janeiro legislative assembly in 1998 after he lost a leg in a bomb attack on his vehicle. Mr. Queiroz, accused of driving the silver Chevrolet Cobalt used in the ambush of Ms. Franco, was expelled from the corporation in 2016 when he was found guilty of providing illegal security services to an illegal gambling house in Rio de Janeiro.

At 4 am this morning, police arrested both men, as well as performing detailed search operations in the apartments of Messrs. Lessa and Queiroz. While scouring the residence of the former, police found dismantled assault rifles alongside ammunition, raising the suspicion that the firearms were intended for sale.

Ronnie Lessa resides in the upmarket neighborhood of Barra da Tijuca, coincidentally in the same housing complex as President Jair Bolsonaro and his son, Carlos. During a press conference on Tuesday, Chief of Police Giniton Lages, responsible for the investigations, stated that one of Mr. Lessa’s daughters dated one of Mr. Bolsonaro’s sons.

Marielle Franco and Anderson’s car

The investigation into Marielle’s killer

Investigators opted to zero in on Mr. Lessa after lifting his digital records and noting that he monitored Marielle Franco’s day-to-day schedule, leading the police to believe that he had been tracking her movements.

Law enforcement became increasingly suspicious of Mr. Lessa after realizing he was the target of a shooting, one month after the assassination of Ms. Franco, in Barra da Tijuca. While driving with a friend, he was shot at by an unidentified individual riding a motorcycle. Investigators believe this may have been an attempt of tying up loose ends by the person or persons who ordered the hit on Marielle Franco.

Investigators agreed to separate the Marielle case into two parts, with one focusing on the perpetrators of the crime (culminating in today’s arrests of Messrs. Lessa and Queiroz), and another looking who gave the order. The police previously said they believed three men were in the vehicle at the time of Ms. Franco’s murder, but this morning have stated there were only two.

Law enforcement believes the assassination was carried out by the so-called “The Office of Crime,” a highly sophisticated and feared death squad linked to the oldest urban militia in Rio de Janeiro. The group, made up of current and former military policemen, uses its insider knowledge and advanced equipment to carry out assassinations to order—we are still in the dark about who was the client in Marielle Franco’s murder.

The assassination

On March 14, 2018, Marielle Franco, 38, a councilwoman for the city of Rio de Janeiro, was on her way home from mediating a debate when her car was cut off by a silver Chevrolet Cobalt, which had been following her for around 4 kilometers. Thirteen gunshots were then fired from the Cobalt. Four of them hit Marielle, three on the head and one on the neck, killing her instantly. Marielle’s driver, Anderson Gomes, 39, was also killed, being shot in the back three times. The Cobalt fled the scene, without stealing any of Marielle or Anderson’s belongings.

All indications showed that the killing of Marielle Franco was a political assassination. A member of the radical left-wing Socialism and Liberty Party, she spent much of her political career denouncing violence committed on behalf of the police and Rio’s militias. A poor, black woman who grew up in the Maré favela in the north of the city, Marielle devoted her life to fearlessly combatting power structures and fighting repression.

In 2008, she worked alongside state lawmaker Marcelo Freixo on a sweeping parliamentary investigation into the actions of militia groups in Rio de Janeiro—the same people suspected to be behind her assassination. The final report of the inquiry led to the indictment of 226 people, and Marcelo Freixo has traveled with bodyguards ever since.

This article was updated on March 12, 2019, at 14:55, to include information given by police chief Giniton Lages.