Staff at London transport hubs have been warned to be vigilant after parcels containing explosive devices were sent to London’s  Heathrow and  City airports as well as the busy Waterloo railway station. A parcel burst into flames after it was opened by staff at an office building on the grounds of Heathrow airport but there were no reports of injuries. The BBC reported the Compass Centre, an office for Heathrow staff separate from the passenger terminals, was evacuated and remained closed several hours after the incident The building was evacuated and specialist officers were called in to make the device safe. A package was also found in the post room at Waterloo but was not opened while the third was found at Civil Aviation House next to London City airport and also remained unopened. Both packages were rendered harmless by police. Some train services were initially suspended but have since returned to normal. London Metropolitan police said the “small improvised explosive devices” were found in A4 sized white postal bags containing yellow Jiffy bags. READ: Ocean Infinity stands ready to resume MH370 search The police said the devices appeared capable of igniting “an initially small fire “when opened and they were treating the incidents as linked. They could not rule out the existence of other bombs and issued advice to transport hubs across London to be vigilant and report suspicious packages to police. Irish police are assisting with the investigation because stamps used on the parcel were from a limited 2018 Valentine’s Day range produced by that country’s postal service and the address “Bus Eireann” package. Bus Eireann is an Irish bus and coach line. The motivation behind the packages is unclear but theories include someone with an Irish Republican grievance or a protest related to Brexit. London mayor Sadiq Khan echoed the police call for vigilance. “While transport services continue to operate as normal, I urge all Londoners and visitors to our city to remain vigilant and report any suspicious packages to police,’’ he said.