Three Vietnamese tourists and an Egyptian guide were killed by a bomb while sitting on a bus heading to a sound and light show at the pyramids in Egypt on December 28.

Authorities say the attack could have been done by a religious group, but there had been no immediate claims of responsibility, Reuters reported. Another 11 Vietnamese and an Egyptian driver were also injured. A total of 14 Vietnamese tourists were on the bus during the attack.

At about 6:15 pm, a bomb hidden near a wall along Mariyutiya Street in Al-Haram district near the Giza Pyramids exploded, causing serious damage to the bus, Egypt’s interior ministry said. Police are investigating, according to a report by The Nation.

Lan Le, a 41-year-old Vietnamese woman, who was on the bus, told Reuters that passengers were screaming after the bomb went off. Le Thi Thu Hang, a Vietnamese foreign ministry spokeswoman, condemned the act of terrorism and asked Egypt to investigate the case.

Mostafa Madbouly, the prime minister of Egypt, told media from Al Haram hospital that the bus had deviated from a route checked by security forces. However, the bus driver said he had not deviated from the normal route.

In July 2017, two Germany tourists were stabbed to death in the Red Sea resort of Hurghada in Egypt.