Police investigating parcel bombs sent to London and Glasgow are linking the packages to those sent to British Army Recruitment Centres.
Detectives want to hear from postal workers who may have come into contact with the devices, sent via post to major transport hubs in the capital as well as to Glasgow University between March 1 and March 22, Scotland Yard said.
On March 5, three packages containing small incendiary devices were delivered to transport hubs in London.
The following day, a fourth package was recovered at the University of Glasgow.
On March 22, a fifth package was recovered, having been returned to a postal depot in Limerick, Ireland.
This package has since been forensically examined and is being treated as part of the same series.
A group calling itself the IRA, known to police as the New IRA, claimed responsibility for the earlier packages and said a fifth had also been sent.
Further analysis of the five packages has been carried out by forensic experts and, due to particular similarities between the devices and the methodology, the investigation teams are now formally linking the packages to those sent to various British Army Recruitment Centres in 2014.