Remy De La Mauviniere / AP, file

Liliane Bettencourt pictured at the Elysee Palace on April 18, 2005.

Former President Nicolas Sarkozy came under formal investigation on Thursday for allegedly taking illegal donations from France’s richest woman when she was suffering from Alzheimer’s disease.

"Nicolas Sarkozy, who benefits from the presumption of innocence, had been notified that he has been placed under formal investigation for taking advantage of a vulnerable person in February 2007 and during 2007 to the detriment of Liliane Bettencourt," the prosecutor in the southwestern city of Bordeaux said in a statement after a hearing, Reuters reported.

Investigating Judge Jean-Michel Gentil is looking into conflicting accounts of how many times Sarkosy visited the home of Bettencourt, the heiress to the L’Oreal cosmetics fortune in the run-up to his 2007 election victory.

Bettencourt, now 90, was judged to be suffering from dementia in 2006. She has since come under the legal guardianship of her family.

Suspicions surfaced three years ago when a former account to Bettencourt made allegations about large donations from her accounts were directed to Sarkozy's campaign.

Patrick Bernard / AFP - Getty Images

Former French president Nicolas Sarkozy, center, leaves Bordeaux's courthouse on Thursday after a hearing before a judge over claims he accepted envelopes stuffed with cash from Liliane Bettencourt.

Sarkozy, 57, lost his immunity from prosecution in May when he was defeated in a bid for re-election by Socialist Francois Hollande.

The ex-president has recently seen a surge of popularity in polls and has hinted at running again in 2017. His supporters say the case against him is politically motivated.

The preliminary charges filed on Thursday mean the investigator has probable cause to believe there was a crime, but he could still drop the charges later.

Even if the charges are not proven, he could be under a cloud of suspicion for months or years.

If he is convicted, Sarkozy could face a prison term of up to three years.

Reuters and The Associated Press contributed to this report.