Fabrício Queiroz is suspected of being part of a scheme to embezzle the salaries of phantom employees in the Rio de Janeiro assembly when Flávio Bolsonaro was a state lawmaker.

He had been on the run but was arrested at a property of a personal lawyer to the Bolsonaro family near São Paulo.

Flávio Bolsonaro denies any wrongdoing.

The case could be embarrassing for the far-right president, who came to office in January 2019 after running on an anti-corruption campaign and promising to be tough on crime.

Jair Bolsonaro is already facing an investigation into an alleged attempt to interfere with the police, and has seen some allies being targeted by a separate inquiry over an alleged fake news scheme.

The political crisis has deepened as the coronavirus continues to spread across Brazil, which has now the world's second-highest number of cases - over 955,000 - and deaths, more than 46,500. Mr Bolsonaro, who initially described the virus as a "little flu", has been heavily criticised for his handling of the outbreak.

Mr Queiroz was arrested in a raid on a property that belongs to Frederick Wassef, a lawyer to the Bolsonaro family, in the city of Atibaia, in São Paulo state. Jair Bolsonaro once said Mr Queiroz had been his friend since the 1980s.

Flávio served as a state legislator for 16 years. Now a federal senator, he has called the investigation a political attack on his family, writing on Twitter: "I face today's events with tranquillity. The truth will prevail! Another piece on the playing board was moved to attack [Jair] Bolsonaro... It's a brutal game!"

The president, his son and Mr Wassef had denied in several occasions having any knowledge of Mr Queiroz's whereabouts. Mr Queiroz's lawyer and Mr Wassef have not commented, and the president is yet to react.

The Supreme Court is already investigating allegations that President Bolsonaro tried to interfere in the work of the federal police for political motives, following accusations made by former Justice Minister Sergio Moro.

Mr Moro, a former judge who oversaw Brazil's biggest anti-corruption investigation known as Operation Car Wash, resigned in April saying Mr Bolsonaro had pressed him to change the chief of the federal police.

He also accused the president of seeking to interfere in investigations that involved family members, and even requested intelligence reports. Mr Bolsonaro has denied the allegations.

A number of Bolsonaro allies including business leaders, bloggers and lawmakers are being investigated in a case that involves an alleged scheme to spread defamatory fake news and threats against Supreme Court members.

A report by Folha de S.Paulo newspaper in April said the federal police had identified another of Mr Bolsonaro's sons, Carlos, as a key member of the scheme. It said the finding was at the centre of the president's attempt to change the police chief. Carlos Bolsonaro has rejected the allegations.

Mr Bolsonaro has urged the Supreme Court to suspend this investigation, saying the justices are politically biased and target his supporters.

And there is a separate investigation into anti-democratic rallies organised by Bolsonaro supporters calling for military intervention in politics and the closure of the Supreme Court and Congress. The president has endorsed some of the protests, and critics say he has sent ambiguous messages about his commitment to the democratic institutions.

Earlier this week, six far-right militants were arrested, including the alleged leader of the group, Sara Giromini, known as Sara Winter. The homes and offices of alleged organisers were also raided.