One of the most iconic paintings in French history, Delacroix's "Liberty Leading the People", has been defaced by a woman allegedly suffering psychiatric problems, but no permanent damage was done.
The 28-year-old woman, who was being held by police Friday, is accused of using a black marker to deface the masterpiece at a recently opened satellite branch of the Louvre in Lens, northern France.
Her motives were unclear but her scrawling of "AE911" had some wondering if she was suffering from delusions involving the 9/11 attacks.
"Is this a person who acted under the influence of some delusion?" asked local prosecutor Philippe Peyroux, adding that the woman had "moments of lucidity but at other times is delirious."
The woman defaced the painting on Thursday before being apprehended by a security guard with the help of another visitor. She was to appear before a judge on Saturday and prosecutors had ordered a psychiatric evaluation.
The Louvre said the painting suffered no long-term damage and that specialists were able to "completely remove" the approximately 30-centimetre (12-inch) mark on the bottom right of the painting.
"The integrity of the work has not been affected, as the inscription was superficial and remained on the varnished surface without reaching the layer of paint," the Louvre said in a statement.
The painting by Eugene Delacroix commemorates France's July Revolution of 1830.
It shows a bare-breasted woman personifying Liberty leading the people forward over the bodies of the fallen, holding the French tricolour in one hand and a bayonetted musket in the other.
The wing holding the painting was closed on Friday but expected to re-open on Saturday.
The woman faces up to seven years in prison and a 100,000 euro ($135,000) fine if convicted of defacing a cultural object, but officials said it seemed she suffered from psychiatric problems.
"It was the act of an unbalanced person," the mayor of Lens, Guy Delcourt, told AFP, praising security for quickly intervening.
"She told security, in a rather incoherent manner, that she wanted to put her mark" on the painting, the mayor said.
The "AE911" mark may indicate her involvement with the Architects & Engineers for 9/11 Truth (AE911Truth), a US group that supports a conspiracy theory that the World Trade Centre was destroyed by an explosive demolition.
Prosecutors did not release the woman's identity, but said she was unemployed, had a master's degree and did not have a previous criminal record.
The Louvre opened the new satellite branch in Lens -- a former mining town plagued by high unemployment -- in December in a bid to revive the region and boost tourism.
Officials said security would be strengthened at the branch following the incident, with more guards expected to be posted.
In 2009 a woman threw a mug of tea at the Mona Lisa at the Louvre's main branch in Paris but it bounced off the protective glass around the painting. In December a Polish man was jailed for two years in Britain for defacing a mural by US artist Mark Rothko at London's Tate Modern gallery.