CAIRO - Two Egyptian police officers were shot dead on Friday by unknown gunmen in the Sinai town of El-Arish in front of a government building, medical sources said, the latest of a string of security incidents in the lawless region.The police officers were guarding the building when the gunmen shot at them, the sources said.Islamist gunmen attacked armed forces guarding the El-Arish airport early on Friday. It was not clear whether the attack, and another on a police station in the nearby town of Rafah, on the Gaza border, were in reaction to the army's overthrow of elected Islamist President Mohamed Morsi on Wednesday.