The Lincoln Memorial in Washington was temporarily closed Friday morning after green paint was found splattered across the floor and on other parts of the statue.
The U.S. Park Police say they were notified of the vandalism around 1:30 a.m. on Friday. The memorial will remain closed until the National Park Service cleans the area.
No words or other markings were found, but an investigation is underway and U.S. Park Police will review surveillance footage from cameras surrounding the statue to try to identify any suspects.
Capt. Steven Booker told The Associated Press that the act "appears intentional based off of the splatter."
Crews are expected to arrive to begin removing the paint on Friday morning, but park officials gave no estimate as to long the memorial will be closed, according to USA Today.
In the past, other national monuments in D.C. have been targets of vandalism, and have sometimes taken weeks to clean up, according to NBC News.
The Vietnam Veterans Memorial, for example, has been vandalized three times, in 1988, 1993 and 2007. In 1988, a swastika and other scratches were found in two panels at the memorial, NBC reported. In 1993, the directory stands at the memorial's entrance were burned. Most recently, in 2007, park police found an oily substance on the memorial wall, which took weeks to remove, The Washington Post reported.
The memorial is typically open 24 hours a day and attracts millions of visitors each year, according to the National Park Service.
Since it was rededicated in 1922, the Lincoln Memorial has been home to many famous protests and events. It commemorates the 16th president of the United States, who emancipated slaves, and serves as a symbol of unity and equality, according to the National Park Service.
In 1939, Marian Anderson, an Afircan-American classical singer, performed on the steps of the memorial on Easter Sunday, after she was denied the opportunity to sing at the Constitution Hall because of her race. She performed for a crowd of more than 75,000 people.
Martin Luther King Jr. gave his historic "I Have a Dream" speech at the memorial following the March on Washington in 1963, when more than 200,000 people traveled to Washington for the civil rights march.