Parcel bombs have been sent to Celtic manager Neil Lennon, a lawyer and a politician, it was revealed on Tuesday night.
Strathclyde Police have launched an investigation after the parcels 'designed to cause harm' were sent to Lennon, Paul McBride QC and Labour politician Trish Godman, sources said.
They were initially thought to be hoax packages but analysis has revealed that they could have worked.
A package addressed to Lennon was intercepted at the Royal Mail sorting office in Kirkintilloch, East Dunbartonshire, on 26 March.
On 28 March a parcel for Trish Godman was intercepted at her constituency office and on 15 April a package was intercepted en route to Paul McBride, who has represented Lennon at Hampden during his dispute with the SFA.
A police source said: 'They were viable devices designed to cause harm. We are treating the matter very seriously.'
Police have offered safety advice to the intended recipients of the packages as well as to Royal Mail staff and workers in the mail room at Celtic.
It is the latest in a series of incidents targeting people linked with the football club.
Earlier this year packages containing bullets were sent to Lennon and to Celtic players Paddy McCourt and Niall McGinn, all three of whom are from Northern Ireland.
Scottish politicians have expressed their shock at the developments.
SNP leader and First Minister Alex Salmond said: 'Let us be quite clear - there is a major police investigation under way to ensure that the individual or individuals concerned are identified and apprehended, and then brought to book with the full force of the law.
'We will not tolerate this sort of criminality in Scotland, and as an indication of the seriousness with which we view these developments the Cabinet sub-committee met last Saturday to ensure that the police investigation has every possible support to come to a successful conclusion. We are confident that this will be the case.
'These disgraceful events should remind all of us who love the game of football of what unites us as a community.
'It is time to remember what we value in society, and unite to condemn those who use football as a pretext for their pathetic and dangerous prejudices.'
