A tourist bus in the Egyptian town of Taba near the Israeli border was struck by an explosion today that killed at least three people and wounded more than a dozen. The targeted bus, which was carrying tourists who had recently been at the famous St. Catherine’s monastery in central Sinai, was preparing to cross into Israel.

At least two South Korean tourists as well as the Egyptian bus driver were killed, according to Reuters. “Almost all 33 passengers on the bus were wounded by the explosion, with 12 suffering serious injuries,” the Associated Press reported. The exact cause of the explosion has yet to be determined, and the attack has thus far gone unclaimed.

Egyptian officials told the Associated Press that they believe the explosion was caused by “either a car bomb or a roadside bomb that was detonated by remote control.” Security sources told Reuters that the explosive device “was planted either inside or near the bus.”