The numbers “8647” — a phrase generally used to signal opposition to President Donald Trump — appeared etched into the grass on the National Mall on Thursday, days before massive crowds are expected in the area for a UFC match on Trump’s birthday on Sunday.

Live webcam images from the top of the Washington Monument show the numbers appearing as dead grass along a large portion of the Mall, east of the World War II Memorial. It’s not clear when the markings first appeared. The numbers were not visible in photos from Getty Images of the National Mall taken on June 5.

The Secret Service deferred comment to the US Park Police, who the agency said was handling the investigation into the markings.

A spokesperson for the Park Police said the cause of the discoloration of the grass has not been determined. Samples were collected for testing, the spokesperson said.

A spokesperson for the Department of Interior, which manages the National Mall, described the markings as “deranged vandalism” which “will not be tolerated” in a statement.

“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 spokesperson said.

The numbers have been used to signal opposition to Trump, though his administration has interpreted them as a threat against the president. In April, former FBI Director James Comey was indicted by the Department of Justice on charges of threatening the president for posting an image on Instagram showing the numbers spelled out in sea shells.

The number “86” is often used as a code in the restaurant industry meaning to get rid of or remove an order or patron, while Trump is the 47th president.

This is a breaking news story and will be updated with more information.