Police in France have detained a woman accused of defacing a painting by Delacroix, Liberty Leading The People, at a provincial branch of the Louvre Museum.
On Thursday, a graffiti tag was scrawled on the painting, a Romantic masterpiece painted in 1830 to celebrate the July Revolution of that year.
"AE911" was written across the bottom of the painting.
The painting is being exhibited for a year in Lens, a former mining town where the museum opened in December.
in a statement, the museum said the mark may be "easily cleaned" but a restoration expert was being sent from the parent museum in Paris.
The gallery was closed to the public on Friday.
"AE911Truth" is the name of a website called "Architects Engineers for 9/11 Truth".