London's Metropolitan Police have released images of three packages sent to transport hubs in the British capital.
Emergency services were called to the three separate incidents at Heathrow Airport, London City Airport and Waterloo train station on Tuesday.
The white envelopes contained an Irish return address as well as Irish stamps.
Gardaí have confirmed they are assisting British police in the investigation.
Police have said the incidents are being treated as a linked series, and investigators are keeping "an open mind" regarding motives.
The packages were all A4-sized white postal bags containing yellow Jiffy bags.
They were assessed by specialist officers to be small improvised explosive devices.
Police said the devices "appear capable of igniting an initially small fire" when opened.
No one was injured in the incidents, but some buildings were evacuated as a precaution.
Commander Clarke Jarrett, head of the Met Police Counter Terrorism Command, said: "The Met Police is working closely with the CT Policing Network and police partners more widely.
"We are pursuing a number of lines of enquiry. One such line is the possibility that the packages have come from Ireland."
"Our utmost priority is to ensure the safety of the public and staff working with mail.
"Yesterday we quickly shared images of the packages with staff at transport hubs and mail handling companies and offices, to aid them in identifying similar packages, however we have not received any further reports."
No arrests have been made.
Main image: The package sent to London's Heathrow Airport | Image: Metropolitan Police