Packages containing bullets have been sent from Northern Ireland to Celtic manager Neil Lennon and player Niall McGinn.
The packages, addressed to the two men, were intercepted by staff at the Royal Mail sorting office in Mallusk, County Antrim, last week.
The mail was bound for Celtic football club in Glasgow.
Police in Northern Ireland said officers were called to Mallusk office on January 5 after postal workers reported suspicions about two items of mail.
The items were removed for further examination, and police are investigating the incident.
A spokesman for Celtic said: "It is now a police matter and the club has no further comment to make."
Police are investigating the incident and have asked anyone with information to come forward.
Celtic winger McGinn is also a Northern Ireland international player.
Lennon, a former Northern Ireland international, has received death threats in the past.