The writing on one of the huge columns at the monument dedicated to slain president Abraham Lincoln, who presided over the abolition of slavery in America, was detected in the early hours yseterday.
The message was difficult to decipher from the photo made available by the park service, but the government agency said it "appears to state '[expletive] law.'"
More indecipherable writing in silver spray paint was discovered on a Smithsonian museum map on Constitution Avenue, which runs along the Mall.
Several treatments will be needed to remove all of the graffiti.
The US Park Police is investigating the incident, which was not the first of its kind this year at Washington's many monuments.
In February, the Lincoln and World War II memorials, as well as the towering Washington Monument, were vandalised with odd messages that included the phrase "Jackie shot JFK," local media reported.
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