“Police are investigating after a package containing a substance was delivered to St. James’s Palace on Monday, February 12,” a spokesperson at the Metropolitan Police tells PEOPLE, confirming that no arrests have been made.
According to the Evening Standard, which first reported the story, the envelope also contained a racist message.
The scare will likely put Scotland Yard on high alert for the couple’s wedding at St. George’s Chapel in Windsor on May 19, where the newlyweds are due to take part in a carriage procession around the streets of Windsor.
Last September, a woman was arrested for breaking into Prince George’s school, and in October, a terror suspect allegedly encouraged ISIS extremists to attack Prince William and Kate Middleton‘s son at school, via a chilling web message.
Kensington Palace declined to comment.