The international criminal inquiry into the incendiary devices apparently sent by post from the Republic to Britain has expanded after a fourth device was found.

The latest alert occurred in Scotland where Glasgow University was evacuated after the device, which is being directly linked to three similar devices in London on Tuesday, was discovered in the college’s mail room.

“The package was not opened and no one was injured,” said Steve Johnson, assistant chief constable at Police Scotland of the Glasgow incident.

“The emergency services were alerted and several buildings within the estate were evacuated as a precaution. A controlled explosion of the device was carried out this afternoon by EOD [explosive ordnance disposal].

“Police Scotland is liaising with the [London] Metropolitan police in relation to their investigation into packages received in London.”

Security sources

While Mr Johnson said on Wednesday it was too early to say if the Glasgow package was linked to the London devices, security sources in Dublin on Wednesday night said London police were treating them as linked.

The three small devices found on Tuesday in London appeared designed to catch fire rather than explode. They were sent to Waterloo rail station and buildings on the Heathrow and London City airport campuses.

A photograph provided by the Metropolitan Police showing a package containing a small improvised explosive device that was delivered to the Compass Centre in London. Photograph: Getty Images

The package sent to the Compass Centre offices at Heathrow was opened, causing the A4 envelope the device was in to catch fire.

However, the other two London packages were not opened.

Images of two of the London packages have emerged publicly, with Irish stamps visible and Dublin as the sender location. The Garda is assisting the London and Glasgow police forces with their investigations.

Dissident republicans

One line of inquiry is that the parcels were all sent from the Republic in the postal service and that dissident republicans were responsible.

However, Garda sources said while the parcels may have been sent by dissidents, they could also be the work of international terrorists, a militant group of some kind or a “lone wolf” attacker, perhaps seeking to generate publicity for an issue.

“It may also be linked to Brexit in some way, perhaps trying to increase tensions between Britain and Ireland, but at this stage we don’t have a fix on that yet,” said one security source in Dublin.

Gardaí added if dissidents were behind the parcels, a statement claiming responsibility would be expected. However, no such statement had been made by late on Wednesday night.

The special detective unit in Dublin, which investigates terrorism, as well as the Garda’s ballistics experts and the Defence Forces’ bomb disposal personnel are set to review the results of tests carried out on the devices found in Britain.

The Garda will also seek to use any postmarks on the packages to try and track their transportation from the Republic to Britain, including their origins in Ireland.

At the University of Essex, an emergency response, and a security cordon, was put in place from 11.50am yesterday following concerns over a suspicious package on the Colchester campus.

However, some 4½ hours later Essex Police said that, on inspection, the package posed no risk to the public.