Three packages rigged as improvised explosive devices, one of which ignited, were found Tuesday near two London airports and a rail station.
Officers discovered that the three postal bags were small IEDs, which appeared "capable of igniting an initially small fire when opened," according to the Met Police Counter Terrorism Command.
Investigators said they are "treating the incidents as a linked series."
The Metropolitan Police received the first report of a suspicious package at Heathrow Airport's administrative Compass Centre. Staff at the building had opened the package, and the device ignited and burned a part of the bag, police said.
No one was injured but the building was evacuated, according to police. No flights were affected.
The second package, which was not opened, was reported from a post room at the Waterloo rail station. The station was evacuated and the package was rendered safe by specialists, according to police. Trains operated normally.
The third package, found at City Aviation House near London City Airport, also wasn't opened. The building was evacuated and the package was rendered safe, according to police. Flights were not affected, but Docklands Light Railway services to the airport were briefly interrupted.
Photos of the packages, which have not been independently verified by NBC News, appear to show Irish postage. Police in Dublin told NBC News that they are assisting in the investigation into the packages.
No arrests have been made, and police encouraged people at travel hubs to be vigilant and report any suspicious packages.