Epifanio Pericolo, aged 28, from Italy, and Denis Labadie, 27, were confined to a Toulon jail on Thursday, although both denied any involvement. Both had cuts and grazes that could correspond to injuries suffered when the assassins who shot Piat on 25 February fell off a motorcycle. Police said they provided alibis.
Police in the port of Hyeres, where Piat was killed as her chauffeur drove her home, said the two men were known to have links with the local underworld. Investigators are still seeking a motive.
Several local politicians, including Joseph Sercia, a vice-president of the Var departmental council, were among 26 people detained for questioning. All but the two presumed motorcycle-riders were released.
Police have turned up two letters written by Piat, who was one of the National Assembly deputies from the Var from the centre-right Union for French Democracy (UDF). One was posted to the local prosecutor and accused Mr Sercia of fraud. The second was marked for opening in the event of her violent death.
Piat, who had gained a reputation for seeking to root out local corruption, was elected for the far right National Front in 1986 but left the party in a dispute with Jean-Marie Le Pen, the president, two years later.
Details emerged to show that the killing was an amateurish affair. Two men fell off their stolen Yamaha motorcycle when they collided with Piat's car and the gunman fired at an angle a good shot would not have attempted, police said.