Detectives have condemned as "despicable and cowardly" whoever sent parcel bombs to Celtic manager Neil Lennon and two prominent fans of the club.
Strathclyde Police said the two packages sent to Lennon, and others to lawyer Paul McBride QC and former MSP Trish Godman, were "designed to cause real harm to the person who opened them".
Two of the four parcels were intercepted at Royal Mail sorting offices in the west of Scotland last month, both addressed to the Celtic manager. The first one was found in Saltcoats, Ayrshire, on March 4 and the second was intercepted on March 26 in Kirkintilloch, East Dunbartonshire.
Two days later a parcel for Ms Godman, former deputy presiding officer of the Scottish Parliament, was delivered to her constituency office.
The most recent package was intercepted last Friday at a post box in Montgomerie Terrace in Kilwinning, Ayrshire, addressed to Mr McBride, who represented Lennon at Hampden during his recent dispute with the Scottish Football Association (SFA).
Detective Superintendent John Mitchell said sending the packages was a "despicable and cowardly act", adding that the initial assessment was that the packages may have been a hoax but confirmed that after forensic examination they are being treated as "viable devices".
Mr Mitchell said: "They were definitely capable of causing significant harm and injury to individuals if they had opened them.
"It is important to say that there is no doubt that there is someone out there with information that can assist us and take this inquiry forward, and the quicker the better."
Speaking at a press conference at Strathclyde Police's Glasgow headquarters, Chief Superintendent Ruaraidh Nicolson urged people who had a "high profile in the media" to be "vigilant" but insisted the general public was not at risk.
Lennon, 39, has endured threats and abuse throughout his career and was forced to retire from representing Northern Ireland in international football after claiming he had received death threats from a paramilitary group.
He was the victim of a street attack in the west end of Glasgow in 2008 and earlier this year received a package containing bullets.
Celtic players Niall McGinn and Paddy McCourt, both from Northern Ireland, were also sent bullets.
Mr McBride is one of Scotland's most-recognised QCs and is a well-known Celtic fan. Last week he accused the SFA of bias in favour of Rangers when assistant manager Ally McCoist and players Madjid Bougherra and El-Hadji Diouf all escaped bans for misconduct during an Old Firm Scottish Cup clash at Parkhead.
The SFA announced later it was considering legal action against Mr McBride for his "unjustified and inflammatory remarks".
Ms Godman, who has stepped down as an MSP, was deputy presiding officer of the last session of the Scottish Parliament as well as Labour MSP for West Renfrewshire. On her final day as an MSP, she was pictured wearing a Celtic football top at Holyrood.
Scotland's First Minister, Alex Salmond, expressed his shock at the "disgraceful events" and said the cabinet sub-committee had met to ensure the police investigation had "every possible support to come to a successful conclusion".
Scottish Labour leader Iain Gray said he was "shocked and appalled" at the "sinister development".
SFA chief executive Stewart Regan said the news was "depressing and deplorable", adding that sectarian hatred was an "unwanted poison" in football.
Mr McBride was unavailable for comment and Ms Godman said she did not want to comment, following police advice.
In a statement on Celtic's website, club chief executive Peter Lawwell said the "vile events" deserved "condemnation from all right-minded people".
He said: "It is an intolerable state of affairs which must end. 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."
Mr Lawwell appealed to Celtic fans to remain calm, adding: "Celtic supporters should rest assured that Neil will continue to receive the full backing of everyone at the club. They should also understand that we will remain strong in the face of such intimidation."
Rangers chief executive Martin Bain said: "We are utterly appalled by the news today that the Celtic manager and others have been subjected to threats which are now the subject of a major police investigation.
"Such behaviour is to be condemned out of hand. These acts have no place in society and no place in football."Reuse content