Three small explosive devices in plastic mailing bags arrived at offices for two London airports and at a train station Tuesday, and Irish police said they were helping British counterterrorism police with the investigation.
London's Metropolitan Police Service said the devices found near London's Heathrow and City airports and at Waterloo Station "appear capable of igniting an initially small fire when opened." A staff member did unseal the bag sent to an office building on Heathrow's grounds, "causing the device to initiate," the police department said.
Part of the package burned, but no one was injured, police said. The Compass Centre was evacuated as a precaution. Flights at Heathrow, Britain's busiest airport, were not affected.
Another mailer turned up at the administration building for City Airport, a much smaller commercial airport. The third was found in the mail room at busy Waterloo Station. Those packages weren't opened.
The Metropolitan Police Counter Terrorism Command "is treating the incidents as a linked series" and "keeping an open mind regarding motives," the department said in a statement.
No person or group has claimed responsibility and no one has been arrested.
Sky News correspondent Alistair Bunkall tweeted a picture of what he said was the package sent to the building near Heathrow. The stamps on the envelope have pictures of hearts and resemble ones issued by Ireland's post office for Valentine's Day 2018.
Police in Ireland told The Associated Press on Tuesday evening they were assisting British investigators, but did not give an explanation or details.
Officials say train services to City Airport were suspended temporarily and the building where the package was found, City Aviation House, was evacuated. Flights were not affected.
Police advised transportation stations throughout Britain to "be vigilant" and to report suspicious packages to police.
The official terrorism threat level throughout Britain is set at "severe," indicating that intelligence analysts believe an attack is highly likely.