The Metropolitan Police said that officers tested a substance in a package received at St James's Palace, which the London Evening Standard said was a white powder. Other UK media reports said the package also contained a racist letter.The investigation is being overseen by the London counter-terrorism command, a Metropolitan Police statement said. Investigators are treating the incident as a "racist hate crime," the statement added.Police would not say who the envelope was addressed to but said tests showed the substance to be non-suspicious. They added than an an allegation of malicious communications is being investigated. St James's Palace is the official London residence of several members of the British royal family including Prince Charles and his sister Princess Anne. The Evening Standard said the package was sent there for sorting before delivery.Prince Harry and Meghan Markle, who announced their engagement in November and are due to marry in Windsor this spring, live at Nottingham Cottage on the grounds of Kensington Palace. Markle is from a mixed-race background.There have been no arrests yet but inquiries continue, according to police. Kensington Palace had no comment.