Egyptian police said on Monday they believe a suicide bomber triggered the blast in a bus that killed three South Korean tourists and the Egyptian driver.
The attack on Sunday, near the Taba border crossing with Israel, was the first targeting tourists since the overthrow of Islamist president Mohamed Morsi in July set off a wave of bombings that have killed scores of policemen and soldiers.
After reviewing CCTV footage of the attack, police concluded a suicide bomber had boarded the tourist bus and detonated explosives near the door.
"The preliminary investigation shows some tourists disembarked to get their bags. A man walked to the bus. There was an explosion when he reached the third step," interior ministry spokesman Hany Abdel Latif told AFP.
The tourists were all members of the same church group from the central South Korean county of Jincheon who were on a 12-day trip through Turkey, Egypt and Israel. They were about to cross into Israel when the attack occurred.
The Al-Qaeda inspired Ansar Beit al-Maqdis group, based in the Sinai peninsula, has deployed several suicide bombers in attacks on police, as well as in a failed attempt to assassinate interior minister Mohamed Ibrahim in September.