Celtic players Paddy McCourt and Niall McGinn are "shocked and disappointed" at having bullets sent to them, their agent said.
Police launched an investigation after a package containing bullets and addressed to McCourt at Celtic Park was discovered on Tuesday by Royal Mail workers in the east end of Glasgow.
It comes days after bullets sent to fellow player McGinn, 23, and Parkhead boss Neil Lennon were detected by postal workers in Mallusk, County Antrim. Lennon, McCourt and McGinn all hail from Northern Ireland where the packages were posted.
Gerry Carlile, the players' representative, said: "I have spoken to both Paddy and Niall and while both are shocked and disappointed, they are determined this won't derail their efforts on the park.
"Both are encouraged by the support and sympathy they have received from both sides of the divide in recent days.
"However, I hope the PSNI (Police Service of Northern Ireland) and Strathclyde Police are investigating thoroughly and that those responsible are caught and brought to justice swiftly."
The three Celtic targets are Catholic and have represented Northern Ireland at international level, but while McCourt and McGinn are favourites of the Windsor Park football ground crowd, Lennon retired from international football in 2002 after receiving death threats.