U.S. Park Police is investigating what it is calling vandalism after the numbers 86 and 47 were found marked in the grass on the National Mall, not far from the site of President Donald Trump’s UFC Freedom 250 event this weekend.

Images from EarthCam showed numbers on the lawn since at least Wednesday afternoon, with the number eight appearing most clearly as browned grass. It’s not clear what caused the grass to brown. 

“86 47” became a contentious phrase after former FBI Director James Comey was indicted for a social media post showing a photo of seashells arranged to say, “86 47.” The government charged Comey with threatening Trump, the 47th president, with the phrase. Comey has denied that he wished any violence against Trump in the post. 

In a statement to ABC News, the Interior Department said that the U.S. Park Police is investigating the incident.

“The deranged vandalism on our National Mall will not be tolerated. Any threat against the President is taken very seriously by the Department, and our U.S. Park Police will investigate this incident and hold those responsible accountable,” the Interior Department said in a statement.

U.S. Secret Service deferred questions to U.S. Park Police, which said that the cause of the discoloration has not been determined but grass samples were collected for testing.

National Park Service workers were seen roping off the area Thursday afternoon, and National Guard members appeared to be directing the public away from the grass. National Park Service workers were seen later roping off the area.