Former French President Nicolas Sarkozy is under formal investigation relating to allegations of corruption.
He is suspected of influence peddling, corruption and benefiting from "the breach of professional secrets," Reuters reports the prosecutor's office as saying.
Mr Sarkozy was earlier held for questions over suspicions he received leaked details of an inquiry into alleged irregularities in his 2007 election campaign
He and his lawyer appeared before a judge on Tuesday evening before being released in the early hours of the morning.
The investigation may be a major blow to his hopes of returning to front-line French politics following his defeat to Francois Hollande in the 2012 presidential election.
Former French President Nicolas Sarkozy is being held for questioning over suspicions he received leaked details of an inquiry into alleged irregularities in his 2007 election campaign.
It is the first time a former head of state had been held for questioning in modern French history and is the latest blow to Sarkozy's hopes of a come-back after his 2012 election defeat by Francois Hollande.
The conservative politician denies all wrong-doing in a string of investigations involving him.
Asked about the matter, government spokesman Stephane Le Foll said Sarkozy was "subject to justice like everyone else."
Former French president Nicolas Sarkozy has been detained for questioning, as part of an investigation into corruption.
Europe Editor James Mates has tweeted:
Former French President Nicolas Sarkozy is being held for questioning, as part of a corruption investigation, Reuters reports.
Sarkozy arrived early this morning to be quizzed by investigators at their offices in Nanterre, west of Paris, after his lawyer was held for questioning on Monday.
"Mr. Sarkozy has been summoned to Nanterre and is being held for questioning," a source told Reuters.
French President Francois Hollande is sitting next to his predecessor Nicolas Sarkozy at the memorial service for Nelson Mandela.
Preliminary charges have been filed against the former French president Nicolas Sarkozy, the AP news agency is reporting.
Mr Sarkozy is being investigated over allegations of taking advantage of a vulnerable person by taking campaign funds from the L'Oreal heiress, which he has denied.
The former French President Nicolas Sarkozy has been placed under formal investigation relating to a case involving L'Oreal heiress Liliane Bettencourt, Reuters has reported.
A judicial source told the news agency Mr Sarkozy was being investigated over allegations of taking advantage of a vulnerable person.
The former French president has repeatedly denied taking campaign funds from Ms Bettencourt, France's richest woman.
Former French president Nicolas Sarkozy has been placed under formal investigation for taking advantage of a vulnerable person, Reuters is reporting.
Nicolas Sarkozy was beaten in the French presidential elections last May and many thought he would disappear into private life.There's been talk he might even move to London to avoid his rival François Hollande's proposed 75% tax on top earners.
I gather that at a dinner in Davos last night he ruled out a move to the UK and hinted strongly that he may return to politics, saying he has more work to do. News perhaps for the current leader of Sarkozy's party, Jean-François Copé.