A big explosion occurred in a security post in the Egyptian town of Rafah destroying parts of the outer wall of the building. However, no casualties were reported, according to eyewitnesses.  Eyewitnesses said they heard the sound of a big explosion in Safa district.   Security sources have said they are now investigating the accident to find out whether the explosion was a result of a rocket-propelled grenade fired at the area or a bomb planted near the wall.  The explosion also destroyed the windows of houses in the Safa district