Federal authorities are investigating after the numbers “86 47” were discovered etched into a grassy section of the National Mall in Washington, D.C., just days before large crowds are expected to descend on the capital for a UFC event scheduled to coincide with President Donald Trump’s birthday.

The markings were spotted Thursday on a sizable patch of grass east of the World War II Memorial. Live webcam images captured the numbers cut into the discolored area, though officials have not yet determined when the damage occurred. Images taken of the same location on June 5 showed no sign of the message.

Although the carving was clearly visible from above, visitors at ground level had difficulty seeing it Thursday afternoon. Witnesses reported that emergency vehicles temporarily restricted access to the area around 1 p.m. while members of the U.S. Army Golden Knights parachute team landed nearby. According to a law enforcement source who spoke with CNN, the Secret Service will assist the U.S. Park Police if investigators identify a suspect.

Authorities are still working to determine exactly what caused the damage. A spokesperson for the Park Police said samples from the affected grass have been collected and are being analyzed. Meanwhile, the Department of the Interior issued a sharp condemnation of the incident, describing it as “deranged vandalism.”

“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,” a department spokesperson said.

The phrase “8647” has circulated in some anti-Trump circles in recent years, while members of the administration have argued that the slogan amounts to a threat directed at the President.

The controversy surrounding the phrase intensified in April when the Department of Justice brought charges against former FBI Director James Comey after he shared a photograph on Instagram showing the numbers arranged with seashells.

The meaning commonly attributed to the phrase stems from the use of “86” as slang for eliminating, removing, or getting rid of something, while “47” is a reference to Trump’s status as the 47th President of the United States.

Investigators have not announced any suspects or motives in connection with the National Mall incident, and the inquiry remains ongoing as authorities work to determine who was responsible for the markings and when they were created.