(Adds Queiroz lawyer, background)
SAO PAULO, June 18 (Reuters) - Brazilian police on Thursday arrested a former aide to Jair Bolsonaro's eldest son in a graft investigation threatening to undermine the president's pledge to crack down on corruption and ratchet up his battle with the judiciary.
Fabricio Queiroz, who worked with Senator Flavio Bolsonaro when he was a Rio de Janeiro state lawmaker, was arrested outside of Sao Paulo in a home owned by a personal lawyer for the Bolsonaro family, according to prosecutors.
Investigators sought Queiroz for questioning over more than 1.2 million reais ($230,000) in bank transactions in a suspected scheme to embezzle the salaries of phantom employees in the Rio state assembly. The senator has denied any wrongdoing and called the investigation a political attack on his family.
"I face today's events with tranquility. The truth will prevail!" Senator Bolsonaro wrote on Twitter after the arrest of Queiroz. "Another piece on the playing board was moved to attack Bolsonaro ... It's a brutal game!"
Speaking later on Thursday, Queiroz's lawyer, Paulo Emilio Catta Preta said his client's arrest was "totally unnecessary" and ruled out him making a plea deal.
The presidential press office and the president's personal lawyer did not immediately respond to requests for comment. However, two sources told Reuters that Bolsonaro had summoned ministers including Justice Minister Andre Mendonca, and the secretary-general of the presidency, Jorge Oliveira, to coordinate a response.
Bolsonaro, a far-right former army captain, ran for office in 2018 pledging to get tough on crime and end decades of political corruption exposed by the record-breaking Car Wash graft investigation.
Since then, critics alleged he has backpedaled on his push to clean up politics, while the president has engaged in an increasingly fraught power tussle with the supreme court that has sparked fears for the constitutional integrity of the country.
Former federal judge Sergio Moro, who oversaw the Car Wash probe and joined Bolsonaro's government last year, quit in April and accused the president of meddling in police appointments for personal reasons, triggering a federal investigation.
The Queiroz case has hung like a dark cloud over Bolsonaro's presidency, raising awkward questions about his and his son's political past in the rough-and-tumble world of Rio politics. Before becoming president, Bolsonaro represented the state as a federal lawmaker for nearly three decades.
($1 = 5.35 reais) (Reporting by Eduardo Simões and Leonardo Benassato Additional reporting by Rodrigo Viga Gaier in Rio de Janeiro Writing by Gabriela Mello Editing by Brad Haynes and Alistair Bell)
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