Parcel bombs sent to London and Glasgow transport hubs earlier this year have been linked to packages sent to British Army Recruitment Centres in 2014.
Scotland Yard has appealed to postal workers who may have come into contact with the devices, which were sent to London Waterloo train station, Heathrow Airport and London City Airport in London, as well as Glasgow University in March this year.
Police are appealing to staff who worked between March 1 and March 22, after three packages containing small improvised explosive devices (IEDs) were delivered to London's transport hubs on March 5.
The package sent to Heathrow was opened by staff and caught fire but no one was injured.
Neither of the improvised explosive devices (IEDs) found at Waterloo and London City Airport ignited, by the airport's Aviation House was evacuated as a precaution.
On March 6, a fourth package was recovered at the University of Glasgow.
Then on March 22, a fifth package was recovered, having been returned to a postal depot in Limerick, Ireland.
The package has been examined and is being treated as part of the same bomb 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.
Analysis of the five packages found similarities between packages sent to various British Army Recruitment Centres in 2014.
Deputy Assistant Commissioner Dean Haydon, senior national coordinator for the UK’s Counter Terrorism Policing, 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.
“The devices that were sent not only put their intended recipients in danger, but also endangered the lives of all those who handled and processed the parcels both in the Republic of Ireland as well as the UK.
“This was an extremely dangerous and reckless act and I would urge anyone who may have information about those responsible to contact police.”
He added: “We are looking to identify any postal workers who may remember handling any of the packages between March 1 and March 22.
“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.”