"We are investigating an allegation of malicious communication received today at QPR FC," said the Metropolitan Police in a statement.
Police would not confirm what was in the letter, which arrived at QPR's training ground on Friday, but local newspaper The Fulham Chronicle reported that it contained a bullet.
QPR play Chelsea in the FA Cup on Saturday, which will be their first encounter since Chelsea captain John Terry was charged with racially abusing Ferdinand during the Premier League game between the sides in October.
Source: AFP
Source: AFP