Richaud said that although the five had been released the investigation was continuing and they could still face charges if new evidence emerged.
He said that for the moment there was "no serious evidence" against Prokhorov, who has a personal fortune estimated at $7.6bn.
The group has been in French custody since Tuesday, when 50 police raided an upmarket hotel in the swish ski resort of Courchevel and arrested 26 people.
Many of those arrested - including eight women in their twenties who said they were models accompanying Prokhorov - were released following the raid.
Prokhorov - who ranks 89th on Forbes magazine's list of the world's wealthiest people - had been staying in Courchevel with a group of Russian businessmen to celebrate the Christian Orthodox New Year.
The police swooped on their hotel after an investigation suggested prostitutes had been procured for guests there.
Pimping is an illegal activity in France even though discreet prostitution itself is legal.
Oleg Bocharov, Moscow's City Duma deputy, was among those arrested in the raid, though he was later released, said Vassily Kuznetsov, the deputy chair of the state Duma's international affairs committee, in the Russian capital.
Russian politicians and media suggested political motives were at play.
A spokesman for Norilsk Nickel in Moscow said: "If Mr Prokhorov is in any way implicated, it can only be a misunderstanding."
The French newspaper Le Parisen reported on Friday that the probe began in February 2005, after the arrest in Courchevel of an Austrian man in possession of a catalogue of call girls.
An employee in a ski shop told the daily that prostitution had long gone hand in glove at Courchevel.
He said: "When you see young women in skimpy clothing going back and forth between the hotels, you have to tell yourself they are not here to go skiing."