Parcel bombs sent to UK addresses including Heathrow Airport could be linked to those sent in 2014 to army careers offices, detectives have said.
“Small improvised explosive devices” were found at Waterloo station, London City Airport and Heathrow on 5 March.
Another was recovered at the University of Glasgow the next day, while a fifth was found after being returned to a postal depot in Limerick, Ireland.
A group calling itself the IRA claimed responsibility for the packages.
Police have now formally linked the latest packages to the seven letter bombs sent to British Army Recruitment Centres five years ago.
That follows further analysis of the packages by forensic experts which has found particular similarities between the devices and the methodology used in both cases.
Met Deputy Assistant Commissioner Dean Haydon said: “Our inquiries continue, but clearly a key element of the investigation now is the link between the devices previously sent in 2014, and the five sent earlier this year.
“As with any investigation, we will be led by the evidence, but at this stage our principal line of inquiry is that the devices were sent by a violent dissident republican group.”
No injuries were caused by either series of packages.
Irish police have been assisting the Met Police as the recent devices – found in white postal bags containing yellow Jiffy bags and delivered to the London transport hubs – all had Republic of Ireland stamps.
The senders’ addresses on the packages were also given as Dublin.
Police said at the time that the packages – the first of which caught fire when it was opened by staff at Heathrow’s Compass Centre – were “not designed to kill” but had “some degree of sophistication”.
Mr Haydon said the police were looking for any postal workers who may remember handling the packages between 1-22 March.
He said: “We have recovered forensic evidence following examination of the devices.
“You may have information that could help us with our investigation and it would also help with our forensic inquiries to be able to eliminate anyone who may have innocently come into contact with any of the five parcels after they were posted.”
Police Scotland said it was working closely with the Metropolitan Police in its investigation.
Assistant Chief Constable Steve Johnson said: “This was a reckless act which caused widespread panic and concern, and while thankfully no one was injured, the disruption it caused to the university’s students and staff in addition to the local community was significant.”