London (CNN) Three improvised explosive devices were found near major London transport hubs on Tuesday, in what police are treating as linked incidents.

The Metropolitan Police's counter-terrorism command unit has launched an investigation into the devices, which were sent in small packages to buildings at or near Heathrow Airport, London City Airport and Waterloo train station.

All the devices were judged to be viable, and in one instance, the package burst into flames when staff opened it. No one was hurt.

The Irish police are also assisting with the investigation, a spokesperson for An Garda Siochana, Ireland's national police force, told CNN. British and Irish news outlets reported that at least two of the packages had Irish stamps.

The Metropolitan Police said that the incidents appeared to be part of a "linked series." Officers were keeping an "open mind" about motive, police said.

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