Packages containing bullets have been sent to Celtic manager Neil Lennon and player Niall McGinn.
The packages, addressed to the two men, were posted in Northern Ireland.
They were intercepted by staff at the Royal Mail sorting office in Mallusk, County Antrim on Wednesday.
The mail was bound for Celtic football club in Glasgow. The PSNI said the items have been removed for examination.
Police were called to the sorting office after postal staff raised concerns about the two packages.
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.
Neil Lennon, a former Celtic captain and Northern Ireland international, has received death threats in the past.
He was forced to withdraw from Northern Ireland's game against Cyprus in 2002 after a reported death threat from a paramilitary group.
In 2008, he was treated in hospital after being assaulted in Glasgow.
Northern Ireland international Niall McGinn transfered from Derry City to Celtic in 2009.
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