LONDON: (DNA) –  UK police say a counter-terrorism investigation has been launched after small improvised explosive devices were found at Heathrow Airport, City Airport and Waterloo Station on Tuesday.

At least two of the parcel bombs sent to the locations had Irish stamps, Irish police are now assisting British counter-terrorism officers, they said on Tuesday.

The mailers – all A4-sized white postal bags containing yellow Jiffy bags – were assessed by specialist officers to be small improvised explosive devices. The devices, at this early stage of the investigation, appear capable of igniting an initially small fire when opened, the Metropolitan Police said in a statement.

The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives. British Transport police said their specialist units were called to Waterloo Station after a “suspicious item” was found in the mailing room, but said the public were not put in harm’s way.

One package was found near Heathrow Airport at an office building, which was evacuated as a precaution after the package was opened and part of it burned causing a small fire. Officials say the building is not at the airport and flights were not affected.

No one was injured by the devices and air and rail services were not affected. No arrests have been made.

The extent of the incident is still not yet known. Most of Waterloo Station remained in operation; when contacted, several shops were not even aware of the incident and one said they were trading as normal. Some were informed, and were given a warning with minimal information, and told to follow updates on the Metropolitan Police’s social media accounts.

The Mayor of London, Sadiq Khan, thanked first responders for their “swift actions” and urged residents and visitors to the city to “remain vigilant.”