The Russian billionaire owner of Monaco football club, Dmitry Rybolovlev, has been placed under formal investigation in connection with a major fraud case.
It involves a Swiss art dealer from whom Mr Rybolovlev bought a series of masterpieces, triggering a long-running separate legal battle.
The investigation centres on claims that Mr Rybolovlev tried to influence senior Monaco officials.
Monaco's former justice minister is among those investigated, reports say.
Under Monaco's criminal system, which mirrors that in France, a suspect is placed under investigation by a magistrate, who then determines whether there is sufficient evidence to hold a trial.
Both Mr Rybolovlev and the former minister deny wrongdoing and no formal charges have been announced.
Monaco's chief prosecutor Sylvie Petit-Leclair said on Wednesday that Mr Rybolovlev was suspected of influence peddling and bribery.
The prosecutor's office told AFP news agency on Thursday that ex-justice minister Philippe Narmino was suspected of influence peddling and accepting bribes. He went into early retirement in 2017 as the case unfolded.
From fertilisers to football
Dmitry Rybolovlev earned his fortune in the fertiliser business, selling his stake in 2010 and then buying the penthouse in La Belle Epoque, a luxury building overlooking Monte Carlo harbour.
He had already bought Donald Trump's Palm Beach mansion in 2008.
In 2011 he bought a majority stake in AS Monaco, a top-flight club in the French league.
The team, under new coach and former player Thierry Henry, crashed out of the Champions League on Tuesday.
There are five suspects in total, AFP reports. All have been released while investigations continue, and they deny doing anything wrong.
AFP said Mr Narmino's wife Christine and son Antoine were also being investigated for alleged conspiracy while Mr Rybolovlev's lawyer Tetiana Bersheda was suspected of collusion.
The long-running dispute involves Swiss dealer Yves Bouvier, whom Mr Rybolovlev accuses of defrauding him of about $1bn (£760,000).
He had purchased 38 works of art from Mr Bouvier over the course of a decade, the prices of which he claims were deliberately inflated. Mr Bouvier denies those accusations.
One of those paintings is Salvator Mundi, believed to be by Leonardo da Vinci, which last year became the most expensive painting ever sold. Mr Rybolovlev auctioned it at Christie's in New York for $450m (£341m).
French reports say the Russian tycoon's lawyer handed prosecutors a phone recording as part of the case against Mr Bouvier. But when investigators looked at text messages on her phone they discovered material that allegedly revealed compromising material on a much bigger scale.
Mr Rybolovlev's lawyers told Reuters news agency that their client was not restricted from travel.
"We particularly insist on the fact that, at this stage, Mr Rybolovlev is presumed innocent," they said.
A spokesman for the Kremlin said Russia was following developments in Monaco with a view to defending the legal interests of its citizens.