Nicolas Sarkozy and Dominique de Villepin were once political rivals
They are investigating claims of a plot in 2004 against his political rival Nicolas Sarkozy, now French president.
Mr de Villepin faces claims that he encouraged the leaking of papers which falsely implicated Mr Sarkozy in a bribery scandal, sources say.
Mr de Villepin has strenuously denied any involvement in the affair.
He has demanded to be declared an "assisted witness" in the inquiry, a move which would enhance his legal rights and give his lawyers access to police files.
He has curtailed a holiday to monitor the search.
A separate six-hour search of his home was conducted in Paris on Thursday.
The alleged scandal is known as the Clearstream affair, after the Luxembourg clearing house in which Mr Sarkozy and other figures were falsely accused of holding secret accounts into which bribes were paid.
He was French foreign minister at the time of the alleged plot, but went on to become prime minister. The claims against him suggest he was hoping to scupper Mr Sarkozy's presidential ambitions.
Justice officials say some information key to the inquiry was allegedly retrieved from the laptop computer of a former military intelligence general.
The computer files were said to support claims that Mr de Villepin initiated a meeting that led to Mr Sarkozy being falsely accused of receiving money in relation to a controversial defence contract - the sale of French frigates to Taiwan in 1991.
Last month, a lawyer for former President Jacques Chirac told Europe 1 radio station that his client would not co-operate with investigators over the Clearstream affair, citing presidential immunity.
Mr de Villepin once harboured presidential ambitions of his own and seemed to enjoy the support of Mr Chirac, before Mr Sarkozy was officially designated as the candidate for the governing UMP party for the 2007 presidential poll.
Mr de Villepin was interviewed last December about the affair.