Bulgarian authorities are investigating the possible causes for a bomb attack that left six Israeli tourists dead and dozens injured on Wednesday (July 18).

The tourists were on a bus in the carpark outside Burgas airport when the bomb went off.

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.

Security has been tightened up around the airport.

Israeli Prime Minister Minister Benjamin Netanyahu has blamed Iran for the attack and said Israel would respond.