Chelsea boss Avram Grant has been the subject of death threats.
The club's training ground at Cobham, Surrey, was sealed off after a member of staff opened the package on Tuesday. A note addressed to Grant claimed the powder was lethal.
A police source confirmed that the note to Grant, 52, included the words: "You are a back-stabbing Jewish b******. When you open this letter you will die a very slow and painful death."
Death threats of a sexual nature were also made to Grant's wife, Tzofit, a well-known television personality in the couple's native Israel.
A spokesman for Surrey Police said: "We were called to Chelsea's training ground at Cobham at 12.15pm yesterday after they received a suspicious package.
"A white powder contained within the package was analysed by specialists from Surrey Fire Service and was determined to be a harmless substance.
"Detectives are continuing investigations to identify the source of the package."
Chelsea were not immediately available for comment on Wednesday.