Celtic Football Club manager Neil Lennon and two other well-known fans have been targetted by "viable" parcel bombs designed to "kill or maim", Scottish police said on Tuesday.
The package addressed to Lennon was thought to be a hoax when found at a sorting office on March 26, but Strathclyde Police said on Tuesday that they would be launching an investigation after it was found to be "designed to cause harm".
Police said they did not believe the bombs were linked to terrorist organisations.
The two other targets were Scottish Labour politician Trish Godman and lawyer Paul McBride, who has previously represented Lennon.
"They were viable devices designed to cause harm," a police spokesman said. "We are treating the matter very seriously."
The package destined for Lennon was found at the Royal Mail sorting office in Kirkintilloch, East Dunbartonshire.
Godman's parcel was intercepted at her constituency office on March 28 while the package sent to McBride was discovered on April 15.
Lennon has been on the receiving end of sectarian threats throughout his playing and managerial career.
Two men were jailed for two years in 2009 after Lennon, a Catholic, was assaulted in Glasgow and earlier in the season the former Northern Ireland international had bullets sent to him in the post.
The ex-Leicester player retired from international football in 2002 after receiving death threats, widely believed to have been made by extremists opposed to a united Ireland.
The Celtic boss was handed a four match touchline ban last month after an altercation with Ally McCoist, coach for Glasgow rivals and mainly Protestant supported Rangers.
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.
"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," argued the Scottish politician.