The arrest on Tuesday of two individuals alleged to be the gunmen who assassinated Brazilian politician and activist Marielle Franco and her driver nearly one year ago is being heralded as progress by human rights campaigners even as they warn this should be seen as the beginning of accountability and justice for the murders, not the end.

According to the Rio Times, the two suspects in custody were identified by police as Ronnie Lessa, a retired military police officer, and Elcio Vieira de Queiroz, who was expelled from the nation's Military Police. Both men are believe to have ties to right-wing militias and criminal gangs often run by ex-police officers and the military.

As the New York Times reports:

The arrests offer the most concrete sign yet that Ms. Franco and her driver, Anderson Gomes, were targeted by members of a criminal underworld run by former and current law enforcement officials. Allies of Ms. Franco have long suspected that such groups, known as militias, may have targeted the councilwoman in retaliation for her activism against police brutality and her defense of human rights. In a charging document, prosecutors said the assassination had been planned for months and asserted that the killing had been motivated by Ms. Franco’s work as a lawmaker.

The arrests come just two days before the one-year anniversary of the slayings that took place in Rio de Janeiro in March of 2018. Erika Guevara-Rosas, the director of the Americas program for Amnesty International, said the brutal murder of Franco, "which devastated the many communities whose rights she fought to defend," was a "blatant attempt to silence a brave human rights defender, who had devoted her life to advocating for women, LGBTI people and black youth in Rio favelas."

In court documents, as O Globo newspaper reports, the Brazilian prosecutors charging the case said, "It is undeniable that [Franco] was summarily executed for her political activities and the causes she defended." And added, "The barbarity committed the night of March 14, 2018, was an affront to a democratic state guided by the rule of law."

U.S. journalist Glenn Greenwald, who lives in Brazil and who knew Franco personally as a friend, responded to the news of the arrests with a series of tweets, including several noting the extremely close and worrying ties at least one of the assassins had to the nation's current right-wing President Jair Balsonaro.

"Nobody yet knows whether the Bolsonaro family had any involvement in Marielle's assassination," Greenwald declared Tuesday morning, "but what we know for certain is that the Bolsonaro family - including the President himself - has multiple links to the militias who did this." Referencing the Brazilian president's upcoming trip to the United States and the support he has received from President Donald Trump, Greenwald noted: "He'll be at the White House in one week."

This is fascinating. One of Brazil's most influential columnists notes that the Bolsonaro family never lamented the assassination of Marielle. In fact, Bolsonaro's militia-linked son, now a Senator, *deleted* a tweet he posted expressing condolensces https://t.co/UMaPGMSDjo https://t.co/21ZpdtJYAA — Glenn Greenwald (@ggreenwald) March 12, 2019

As the day progressed, however, the number of connections continued to build and Greenwald tweeted this with a heavy hint of sarcasm:

So odd that such a clean, upstanding family like the Bolsonaros - who hate crime & corruption so deeply - ended up having so many close friends, employees & associates who run Brazil's worst militias & got rich murdering people. And oh: here's Bolsonaro with Marielle's assassin: pic.twitter.com/3Kw7zI85Tl — Glenn Greenwald (@ggreenwald) March 12, 2019

In her statement, Amnesty's Guevara-Rosas said Tuesday's arrests were "the first sign of progress in an investigation that has barely moved in the year since the killings," and said her group is calling for the Brazilian authorities to ensure the ongoing investigation into the killings is "independent and impartial" and that it ultimately brings "all those responsible, including those who ordered the crime, to justice in fair trials."

Greenwald agreed that as important as arresting the men who possibly carried out the crime was identifying and bringing to justice the much more powerful people who likely ordered the killings. "The key point: this is just the first step in this case, not the last," he stated. "These police officers were paid assassins. They had huge amounts of assets. They worked for a militia. The only question that matters now is: who ordered them to kill Marielle & why? Lots of power involved."

According to the NY Times, Wilson Witzel, the governor of Rio de Janeiro, told reporters the suspects now in custody may be persuaded to strike a deal with prosecutors in exchange for information that "leads authorities to build a case against the masterminds of the crime."