The memorial to President Abraham Lincoln, who led the US through the Civil War, has been vandalised in Washington DC.
The vandals appear to have used red spray paint to write "[expletive] law" early on Tuesday morning, the National Park Service said in a statement.
Crews are removing it with a "gel-type architectural paint stripper that is safe for use on historic stone".
US Civil War memorials have become a lightning rod in the US race debate.
Other "undecipherable graffiti" was found written in silver paint on a nearby sign directing tourists to the Smithsonian Institution on Washington's National Mall public park.
President Lincoln led the northern Union government to defeat the secessionist Confederacy and ordered African-American slaves to be freed in 1863.
He was assassinated in Washington only five days after southern General Robert E Lee surrendered, effectively ending the war.
A statue of General Lee became a flashpoint over the weekend, when white supremacists clashed with anti-racist protesters in Charlottesville, Virginia, over that monument's removal.
A protester died after she was run over, and President Donald Trump was widely criticised in US media for failing to immediately lay blame for the violence on the Ku Klux Klan and neo-Nazi groups.
Statues to Confederate leaders have been targeted for removal by many cities across the US, including Charlottesville, following a 2015 hate attack by a man in South Carolina who used the rebel flag to espouse his racist views.
After Saturday's violence several more cities announced plans to remove their monuments to the Confederacy.
Baltimore, Maryland; Lexington, Kentucky; Memphis, Tennessee; and Jacksonville, Florida have all announced proposals to remove controversial monuments.
On Monday, protesters in Durham, North Carolina, toppled a bronze Confederate statue as police filmed the crowd.
Durham County Sheriff Mike Andrews later issued a statement saying police are reviewing the video in an effort to identify and prosecute those responsible for bringing down the 1924 statue.