Police investigating a series of bombs sent through the post, including to London City Airport, are appealing for mail workers to come forward as they attempt to find who is behind them.
A package containing a small incendiary device was delivered to Aviation House, a staff-only building at the Royal Docks airport, on March 5.
On the same day, similar packages were delivered to a building near Heathrow Airport and to Waterloo station's postroom.
All the packages were A4-sized white postal bags containing yellow Jiffy bags and appeared capable of igniting a small fire when opened, according to the Met Police.
The next day, a fourth package was recovered at the University of Glasgow while a fifth - intended for Charing Cross station - was recovered later that month after being returned to a post depot in Limerick, Ireland.
Police are treating the bombs - which a group calling itself the New IRA calimed responsibility for in March - as linked.
Detectives from four police forces - the Met and its counterparts in Scotland, Northern Ireland and the Republic of Ireland - are now appealing for anyone who has information about the origin of the devices or who handled them to come forward.
Deputy Assistant Commissioner Dean Haydon said: "We are looking to identify any postal workers who may remember handling any of the packages between March 1 and March 22.
"You may have information that could help us with our investigation and it would also help with our forensic enquiries to be able to eliminate anyone who may have innocently come into contact with any of the five parcels after they were posted."
He added that investigators were now formally linking the bombs to packages sent to British Army recruitment centres in 2014, due to similarities between the devices.
"As with any investigation, we will be led by the evidence, but at this stage, our principal line of enquiry is that the devices were sent by a violent dissident republican group," he said.
"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.
"This was an extremely dangerous and reckless act and I would urge anyone who may have information about those responsible to contact police."
Anyone with information should call police on 0800 789 321, or Crimestoppers anonymously on 0800 555 111.
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