Police have confirmed they are linking a package received by Glasgow University to those posted to three London transport hubs earlier this week.
Several university buildings were evacuated and nearby roads closed the device was discovered the Wednesday morning. The device was detonated and there were no injuries, Police Scotland confirmed.
The Metropolitan Police says the package contained a "similar-type device" to those sent to Heathrow Airport, London City Airport and Waterloo railway station on Tuesday. It confirmed it is working with Police Scotland to share information on the investigation.
The packages reported in London on Tuesday were posted with Irish stamps and had Dublin as the return address, prompting Irish police to join the investigation. It is not know if the package sent to Glasgow University has the same postmark.
Commander Clarke Jarrett, from the Met's Counter Terrorism Command, said: "Due to similarities in the package, its markings and the type of device that was recovered in Glasgow, we are treating it as being linked to the three packages we're investigating in London.
"We are working very closely with our colleagues from Police Scotland and both investigations are being run in tandem.
"We continue to pursue a number of lines of enquiry and one such line is the possibility that the packages have come from Ireland. However, we are still keeping an open mind with regards who may be responsible and any possible motivation.
"Our utmost priority is to ensure the safety of the public and staff working with mail. We have issued extensive advice to transport hubs and mail sorting companies to be vigilant for and report suspicious packages to police.
"Following today's developments in Glasgow we have extended this advice to others including reiterating it to those in the education sector. As always we encourage anyone who sees something suspicious to report it to police immediately."
Security sources suggested the packages’ Irish insignia may have been a “concerted attempt” to make them appear as though they were posted from Ireland, but could not rule out that they had been.
All were A4-sized white postal bags containing yellow Jiffy bags and appeared capable of igniting a small fire when opened.
The stamps appeared to be those issued by the Irish postal service for Valentine’s Day 2018, featuring a heart motif and the words “Love Eire N”.
The senders’ addresses were given as Dublin, with two having adding coach operator Bus Eireann, a subsidiary of a company accountable to the Irish government.
Ireland's Foreign Minister Simon Coveney has condemned the packages, saying he received the news they may have come from Dublin with "dismay."
The UK's terror threat level will not be increased, the government said on Wednesday.