Brazilian police have arrested former justice minister Anderson Torres, a close ally of recently defeated President Jair Bolsonaro.
Torres was in charge of public security of the capital, Brasilia, when government buildings came under attack on January 8.
About 4,000 Bolsonaro supporters broke into and vandalized the Congress and other buildings, including the presidential palace. More than 1,300 people have been arrested.
Police arrested Torres on Saturday when he returned to Brazil from the United States.
Torres is reportedly suspected of intentionally failing to act to stop the capital attack despite having knowledge of the plan. He left the country before the riot.
Local police say they found a draft decree in Torres' home overturning the results of a Bolsonaro loss in the October presidential election.
Torres wrote on his Twitter account that he had always acted ethically and legally, and that he believes the truth will prevail.
Prosecutors are expected to question Torres in connection with the former president's alleged involvement in the riot.