French police have detained a woman accused of defacing an iconic Delacroix painting, Liberty Leading The People, at a branch of the Louvre Museum, reports BBC News.
She was held after being seen scrawling a graffiti tag on the painting, a Romantic masterpiece painted in 1830 to celebrate a French uprising.
The museum in the northern town of Lens said the work might easily be cleaned but would be examined by a restorer.
The Louvre Lens museum only opened in the former mining town in December.
The painting by Eugène Delacroix, which featured on the pre-euro, 100-franc French banknote and reportedly inspired the Statue of Liberty in New York, is being exhibited in Lens for a year.
French media quoted unnamed legal sources as saying the graffito was a clear reference to a 9/11 conspiracy theory.
The gallery remained closed to the public on Friday.
Just before closing time the previous day, a 28-year-old woman scrawled the 30cm (12in) graffito on the bottom of the painting and was immediately detained by a museum guard, France's 20 Minutes news website reported.
The work itself, which commemorates the July Revolution of 1830, measures 325cm by 260cm.
The mark may be "easily cleaned" but a restoration expert was being sent from the parent museum in Paris, the museum said in a statement.
Read more of this report from BBC News.