Bulgarian authorities are investigating the possible causes for a bomb attack that left six Israeli tourists dead and dozens injured on Wednesday (July 18).<br/> <br />The tourists were on a bus in the carpark outside Burgas airport when the bomb went off.<br/> <br />Police say they are trying to determine whether the explosives were put on the bus before the tourists boarded, or if they were inside a passengers luggage.<br/> <br />Security has been tightened up around the airport.<br/> <br />Israeli Prime Minister Minister Benjamin Netanyahu has blamed Iran for the attack and said Israel would respond.
