The four parcel bombs sent to Celtic manager Neil Lennon and two prominent supporters of the Glasgow club were live devices that could have caused "significant harm," police said Wednesday.
The devices were sent in the weeks after a tumultuous match between Celtic and fierce Glasgow rival Rangers, two clubs with a history of sectarian conflict. The packages were intercepted before reaching their targets and did not explode.
Detective chief superintendent John Mitchell of Strathclyde police said, after initial suspicion that the packages may have been a hoax, forensic tests showed they were "viable devices."
"They were definitely capable of causing significant harm and injury to individuals if they had opened them," he said.
While police didn't discuss the motive behind the mail bombs, sectarian tensions in Northern Ireland and Glasgow are regularly played out between Celtic fans, who are mostly Catholic, and Rangers fans, who are mostly Protestant.
The first parcel bomb targeting Lennon, a Catholic from Northern Ireland, was found on March 4 and a second was intercepted at a sorting office outside Glasgow on March 26.
Another package destined for Celtic-supporting Scottish lawmaker Trish Godman was intercepted at her constituency office two days later. A fourth package destined for Paul McBride, a lawyer who has represented Lennon, was intercepted earlier this week.
Celtic chief executive Peter Lawwell condemned the "repeated threats and intimidation."
"It is an intolerable state of affairs which must end," he said. "Celtic, from our inception, has been a club open to all. We enjoy friendship and respect throughout the world yet, here in Scotland, we are caught up in these vile events."
Stewart Regan, the English chief executive of the Scottish Football Association, said the sport "must not be used as a platform for religious intolerance or hatred."
"As a relative newcomer to this country, I find this recent situation both depressing and deplorable," Regan said. "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."
Scotland's First Minister Alex Salmond called for a "renewed determination that our beautiful game should not be besmirched" by the latest incidents.
"What is required here is that all people — particularly those that love the game of football — unite to condemn such actions and unite to condemn those who abuse football for their pathetic and dangerous prejudices ... that belong in a long and distant past," Salmond told the BBC.
UEFA last week hit Rangers with a second charge of "discriminatory behavior" over its fans' sectarian chants during Europa League matches against PSV Eindhoven last month.
UEFA President Michel Platini said Wednesday that soccer has to work together to solve the problem.
"We will fight against all violence, sectarianism, everything we will fight," Platini said during a visit to London. "Football is very nice, it's a beautiful game and you have many beautiful things. But you have some bad things. As UEFA president you have take responsibility to look what are the bad things and change the system."
The news comes just days before Rangers hosts Celtic on Sunday in the teams' seventh meeting this season. Rangers are trying to close a four-point gap on the defending champions at the top of the Scottish Premier League.
The parcel bombs are the latest in a series of incidents targeting Lennon and others connected with Celtic.
Packages containing bullets were sent earlier this year to Lennon and to players Paddy McCourt and Niall McGinn, who are also from Northern Ireland.
The 39-year-old Lennon has endured threats and abuse during his soccer career — both as a player and a manager.
Lennon quit international soccer in 2002 after having made 39 appearances, saying he had received death threats from a paramilitary group in Northern Ireland.
AP Sports Writer Rob Harris in London contributed to this report.