A suspect package sent to an Irish republican organisation is similar to those sent to Celtic manager Neil Lennon and two other high-profile supporters of the club, police said.
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.
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 Northern Ireland. 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."
Police are looking for a young couple who were seen in Kilwinning on Friday April 15. Both were last seen in Montgomerie Terrace at 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."