Authorities are investigating apparent discolorations on the grass of the National Mall that may spell out “86 47,” a set of numbers which Donald Trump’s Administration has claimed to be a threat or allusion to the assassination of the President.

Aerial images taken Thursday, just days before Trump’s birthday on June 14 and the Ultimate Fighting Championship (UFC) event at the White House, show the apparent staining in the grass near the World War II Memorial. The photos show a visible 8 and traces of 6, 4, and 7.

It wasn’t immediately clear when the markings first appeared. The Washington Post reported the numerals were already visible on Tuesday. Reuters reported that one of its photographers saw the markings shortly before the U.S. Park Police and ​members of the National Guard arrived on scene. They were also visible from a live cam atop the Washington Monument. 

Merriam-Webster says “86” is restaurant industry-derived slang which means “to throw out,” “to get rid of,” or “to refuse service to,” while noting that it has also recently been used to mean “to kill.” Meanwhile “47” may be used as a reference to Trump, the 47th U.S. President.

The numerals have been used together in the past to signal opposition to Trump. One notable use was when former FBI Director James Comey, an outspoken critic of the President, posted on Instagram last year an image of seashells arranged to show “86 47.” Comey was indicted in April for allegedly making a threat against the President; he has maintained innocence and vowed to fight the case, citing free speech.

The Interior Department called the latest incident at the National Mall “deranged vandalism,” adding that “any threat against the President is taken very seriously” and that U.S. Park Police will investigate “and hold ​those responsible accountable.”

In statements to the media, a spokesperson for the U.S. Park Police said ‌they haven’t determined cause ⁠of the "discoloration,” adding that a probe is underway and grass samples have been collected for testing.

White House spokesperson Davis Ingle said in a separate statement to the media Thursday that “anyone who engages in or endorses political violence or assassination culture must be condemned in the harshest terms possible” and that “they should also immediately seek psychiatric help to treat their severe and debilitating case of Trump Derangement Syndrome that has warped their brains and made them sick in the head.” 

The National Mall has been the site of previous demonstrations against Trump. Last year, a statue of the President holding hands with the late convicted sex offender Jeffrey Epstein appeared. During the most recent “No Kings” protests in March, demonstrators marched from Arlington, Va., to the National Mall to denounce the President.

Earlier this month, a federal judge stopped the National Park Service from taking action against the progressive group Accountability NOW USA. The group displayed an “86 47” flag during a permitted demonstration near the George Meade Statue in D.C., where they called for Trump’s removal from office. The display prompted an investigation by the Secret Service.

But according to District Judge Randolph Moss, “it is difficult to fathom how the NPS (or the Secret Service) could have concluded that a reasonable observer would view the flag as a true threat.”