“Liberty Leading the People” by Eugene Delacroix. Click on the photo to see it full-size.

The 29-year-old woman used an indelible maker to write “AE911” on the 1830 painting, one of the best known symbols of the 18th-century Gallic upheaval that overthrew the monarchy.

Click here or photo to see the full painting!

Its most famous feature is a bare-breasted woman who is the personification of liberty. She’s holding aloft the French Tricolor with a musket in the other.

The painting is so adored in France it was used on its 100-franc bank notes from 1978 to 1995, according to French news service France-24.

The tagging occurred late Thursday during the museum’s regular hours. The painting, readily accessible to visitors, hangs in a satellite facility of the Louvre in the city of Lens. It’s only been opened since Dec 4.

The inscription is reportedly short for “Architects and Engineers for 9/11 Truth.” It’s a group of conspiracy theorists who believe the government played a role in the collapse of the World Trade Center twin towers in New York City. The buildings were struck by airliners during the Sept. 11, 2001 terror attack.

Last October, a Mark Rothko painting was defaced at London’s Tate Modern museum, in an incident that also involved “tagging.”

Museums have been grappling with how much security to provide for priceless works of art without compromising their mission to make works accessible to the public.

TheImproper wrote about an incident in Jan. 2010 involving a woman who tore a hole in a Picasso at the Metropolitan Museum of Art in New York.