Brazilian investigators, taking stock of the damage around the capital and questioning detained protesters, face several major questions as they piece together how rioters briefly seized the seats of Brazil’s government.
The protesters, supporters of the far-right former president Jair Bolsonaro, had been camping out since he lost October’s election to Luiz Inácio Lula da Silva. Mr. Bolsonaro had asserted without any proof that Brazil’s election systems were rife with fraud, but he agreed to a transition of power to Mr. Lula after the election.
The pro-Bolsonaro protesters claimed the election was stolen, though Brazil’s military and independent experts found no credible evidence of voter fraud.
Now, investigators will focus in part on how the demonstration on Sunday was organized, and on how it transformed into a violent riot. In the days after Mr. Lula took office on Jan. 1, there were widespread calls on social media for a huge demonstration in the capital, Brasília.
Those calls circulated among supporters of Mr. Bolsonaro mostly on two apps, WhatsApp and Telegram. Some messages urged people to organize attacks against critical infrastructure, such as oil refineries and roadblocks. On Telegram, some called for the storming of the Monumental Axis, the avenue that goes directly to major government buildings.
According to an intelligence briefing by the military police of Brasília, at least 100 buses carrying 4,000 demonstrators arrived between Friday and Sunday. It was not immediately clear where the social media calls first originated, or how the caravans of buses were organized.
Most of the people who arrived in recent days stayed in an encampment in the capital that supporters of Mr. Bolsonaro had maintained in front of the army’s headquarters since the election in October.
It was also not clear why the rioters were able to breach government buildings — Congress, the Supreme Court and the presidential offices — so easily. State police officers had tried to repel them, but they were far outnumbered.
Videos showed protesters wandering and ransacking the halls of power. Federal officials later distributed images and videos that showed destroyed computers, art ripped from frames and firearm cases without their guns.
Eventually, the military retook control of the buildings, and the authorities began to make arrests. The authorities had arrested at least 200 people as of late Sunday, according to Brazil’s justice minister, though Ibaneis Rocha, the governor of the district that includes Brasília, said that evening more than 400 people had been arrested.
At least 1,200 people were detained for questioning, a police spokesman said on Monday. Some could be charged with committing crimes against democratic institutions or with attempting to unseat a democratically elected government, he said.
Overnight, a Supreme Court judge suspended Mr. Rocha, a supporter of Mr. Bolsonaro’s re-election campaign, for 90 days while investigations take place into security failures. Mr. Rocha on Sunday called the riots an act of terrorism, and said on Twitter that the hundreds of people arrested in the aftermath would “pay for the committed crimes.”
Mr. Lula signed an emergency decree late Sunday that put federal authorities in charge of security in Brazil’s capital, and the dismantling of protest camps has since proceeded peacefully. Brazil’s Congress was called back from recess for an emergency session.
Mr. Bolsonaro, who appeared to be in Florida, criticized the protests on Sunday evening, saying on Twitter that peaceful demonstrations were part of democracy, but that “destruction and invasions of public buildings, like what occurred today,” were not. He also repudiated Mr. Lula’s comments that he bore some responsibility for the riots, saying those accusations were “without proof.”