Scottish Premiership side Kilmarnock have contacted police after receiving a letter on Monday containing "racist abuse" directed at manager Alex Dyer, the club have said.
The 55-year-old Englishman is the only Black manager in the Scottish top flight.
"We are working with Police Scotland to identify any individuals involved in this disgusting act and we will pursue the strongest possible action we can against them," the club said in a statement.
"As a club, we have shown our support to rid the game and society as a whole of racial discrimination by taking the knee before each fixture this season and we will continue to do so.
"Racism in any form is completely unacceptable and must be called out whenever and wherever it is found."
Kilmarnock's 2-1 loss to Livingston on Saturday was their fifth successive league defeat.
(Reporting by Greg Stutchbury in Wellington; Editing by Peter Rutherford)
