Parcel bombs have been sent to Celtic manager Neil Lennon, a lawyer and a politician.
Strathclyde Police in Scotland 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 March 26th.
On March 28th a parcel for Trish Godman was intercepted at her constituency office and on April 15th 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." Mr McBride said he did not want to comment.
Ms Godman, who is stepping down as West Renfrewshire MSP, did not want to comment either as police have advised her not to.
She has been pictured wearing a Celtic top at the Scottish Parliament, the BBC reported. Strathclyde Police are investigating and 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.
Lennon (39) has endured threats and abuse during his football career. He signed for Celtic in 2000, before retiring from the game and later becoming coach and then manager.
He was the victim of a street attack in the west end of Glasgow in 2008. Lennon 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.
Scottish politicians have expressed their shock at the latest developments. SNP leader and Scottish 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."
Scottish Labour leader Iain Gray said: "I am shocked and appalled at this sinister development.
"It is vital though that people keep calm and do not let the situation spiral out of control as that would be playing into the hands of the extremists behind this vile act. It is outrageous that something like this should happen in Scotland in this day and age. Strathclyde Police must have whatever support they need."
A spokesman for Celtic chairman, Lord John Reid, said the club had been kept fully informed on events and should be making a statement on the matter later today.
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: "The Scottish FA is horrified and saddened by the news that the Celtic manager, Neil Lennon, has been subjected to another - reportedly 'viable' - threat along with two other prominent supporters of the club.
"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.
Regan, originally from County Durham, added: "On a personal note, as a relative newcomer to this country, I find this recent situation both depressing and deplorable.
"With the support of the police, the Scottish Government and our other league bodies, it is our intention to help rid Scottish football of this unwanted poison which seems to be prevalent in society."
PA