Parcel bombs have been sent to Celtic manager Neil Lennon, a lawyer and a politician.
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 on March 26. On March 28, a parcel for Ms Godman was intercepted at her constituency office and on April 15 a package was intercepted en route to Mr McBride.
A police source said: "They were viable devices designed to cause harm. We are treating the matter very seriously."
Ms Godman, who is stepping down as West Renfrewshire MSP, had been pictured wearing a Celtic top at the Scottish Parliament, the BBC reported. Mr McBride has represented Lennon at Hampden during his dispute with the SFA.
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.
Lennon, 39, has endured threats and abuse during his football career. The former Celtic player won 39 caps for Northern Ireland, but announced his shock retirement from international football in 2002 after claiming he had received death threats from an Ulster paramilitary group.
The chief executive of the Scottish Football Association said the news was "depressing and deplorable".
Stewart Regan said the SFA was "horrified and saddened" by the attacks, adding that sectarian hatred was an "unwanted poison" in football.
He said: "It is hoped that swift action by Strathclyde Police will succeed in capturing the perpetrators of these cowardly acts and bring the full weight of justice down on them. Scottish football should be a safe and entertaining environment for players, coaches and supporters. It must not be used as a platform for religious intolerance or hatred."