A suspect package sent to a republican organisation is similar to those sent to Celtic manager Neil Lennon and two other high-profile supporters of the club, police said today.
The package was intercepted by police investigating parcel bombs sent to the Celtic manager and one each to lawyer Paul McBride QC and former Labour MSP Trish Godman.
Today Strathclyde Police confirmed that a fifth package was addressed to the offices of Cairde Na H'Eireann (Friends of Ireland) in Glasgow, possibly entering the postal system in Scotland.
The Scotland-based group organises republican marches, including the James Connolly march through Edinburgh and the Bloody Sunday march every January in Glasgow. It also campaigns against racism and sectarianism in Scotland.
The package was discovered at the National Return Letter Centre in Belfast about two weeks ago but nothing suggests a link to the North.
Officers from the Police Service of Northern Ireland recovered the package and alerted the Strathclyde force.
Early investigations suggest it is similar to the ones already being investigated, and was designed to cause "fear, alarm and real harm", police said.
Chief Superintendent Ruaraidh Nicolson, of Strathclyde Police, said: "Naturally, the public are going to be concerned about the latest discovery. However, we would like to emphasise that this device is not a new one. It entered the postal system at the end of March, around the same time as two of the other packages.
"Like the others, it was addressed to individuals (or) organisations that have experienced high-profile media attention lately and it was certainly a viable device.
"We continue to have daily dialogue with Royal Mail, and their continued vigilance has been invaluable.
"The support and co-operation from the public has also been extremely positive. However, I would like to stress that the answers lie out there and we need as much information from the wider public as possible.
"Someone out there knows who is behind this irresponsible and appalling crime, and we will continue to work round the clock until the individual or individuals are brought to justice."
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 package addressed to Mr McBride was intercepted at a postbox in Montgomerie Terrace in Kilwinning, Ayrshire, on April 15.
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 attacked in the street 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 the North, were also sent bullets.
Mr McBride is one of Scotland's most-recognised QCs and is a well-known Celtic fan.
Ms Godman, who has stepped down as an MSP, represented West Renfrewshire. On her final day as an MSP, she was pictured wearing a Celtic football top at Holyrood.
Police are looking for a young couple who were seen in Kilwinning on Friday April 15.
Both were last seen in Montgomerie Terrace around 2.10pm boarding a Stagecoach number 20 bus which travels to Ardrossan. Both got off the bus at Kilwinning railway station a short time later and boarded a train to Glasgow Central station.
The man is white, in his early 20s and has dark-coloured hair. He was wearing a distinctive light-blue and white top with horizontal stripes and jeans.
The woman is also white, in her early 20s and was wearing a dark-coloured padded jacket.
Mr Nicolson added: "It is imperative that this couple come forward. I would like to stress that they are merely witnesses. However, any information they have could be helpful to the investigation."