An explosion ripped through a bus near a border crossing between Egypt and Israel in the Sinai Peninsula on Sunday. The blast killed at least four South Koreans tourists and the Egyptian driver, according to security officials.

The officials the tourist bus arrived at the Taba crossing from an ancient monastery in central Sinai.

Almost all 33 passengers on the bus were wounded by the explosion and were being treated in hospitals in Egypt and across the border in the Israeli port city of Eilat.

The officials said the source of the explosion was not clear, but they believe it was either a car bomb or a roadside bomb that was detonated by remote control.