CAIRO: The loud explosion that residents of Cairo and Giza heard today was the sound of an aircraft that broke the sound barrier above Cairo, said security sources in Egypt's Interior Ministry. They said the aircraft took off from Helwan airport and is related to a sovereign authority in Egypt. Cairo and Giza residents heard a sound like an explosion this morning. It raised a state of panic among Egyptian citizens. Many citizens though they heard a loud explosion, but security officials discovered the true source of the sound.