Three firebombs found at major London transport hubs have sparked fears of a possible Irish dissident plot.
The devices were discovered at Waterloo rail station, London City Airport's headquarters and Heathrow Airport on Tuesday.
Images showed the remains of the package sent to Heathrow and the envelope for the explosive device which was sent to Waterloo.
Both have stamps featuring red hearts that are believed to originate from Ireland.
Irish police confirmed its officers are assisting Scotland Yard.
Sources close to the terror investigation said it was too early to speculate on who was behind the parcel bombs.
But dissident Republican terrorism could not be discounted.
A source told The Mirror that it is thought the firebombs were a “warning” from the New IRA.
Speaking to the newspaper, they said: “The New IRA are the prime suspects and their inner core is very tight.
“It is thought they are using the skills of older experienced people to create mayhem.”
Scotland Yard confirmed the device sent to the Compass Centre near Heathrow Airport was opened by staff, "causing the device to initiate".
"This resulted in part of the package burning," the force added in a statement.
No one was injured.
Police said: “The building was evacuated as a precaution. Specialist officers attended and made the device safe.”
The incidents occurred within two hours of each other and sparked evacuations and road closures.
The first package was discovered at 9.55am at The Compass Centre in Hounslow.
British Transport Police then responded to reports of a suspicious package at Waterloo station at 11.40am.
The Met Police responded to another package at City Aviation House at London's City Airport in Newham half an hour later.
The force said: “Staff were evacuated from the building. Specialist officers are at the scene. Enquiries are ongoing.”
