A counterterrorism investigation is underway by London police after three improvised explosive devices were found across the city on Tuesday.

All of the packages were white postal bags containing yellow bags inside them which had material that the police said was capable of igniting "an initially small fire when opened." No arrests have been made. "The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives," a release said.

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The first device was found at a building near Heathrow Airport in southwest London at around 9:40 a.m. U.K. time. The package ignited after staff in the building opened it.

Then shortly before midday, British Transport Police were called to reports of a suspicious package in the post room at Waterloo rail station. Specialist officers made the device safe.

Just moments later, police were called to a report of a suspicious package at offices at the City Aviation House by City Airport in east London. The building was evacuated as a precaution. The device was made safe by specialist officers.

No injuries were recorded and no flights were delayed although police closed the light rail service to City Airport for a short time.