At least 257 people were killed when a military plane crashed soon after takeoff in northern Algeria on Wednesday.The cause of the crash was unclear, and an investigation has been opened. More than 300 emergency workers were working at the scene, which was in a field near the Boufarik military base.Authorities said the Soviet-designed Il-76 military transport plane was carrying soldiers.The flight had just taken off from Boufarik, which is located about 20 miles southwest of Algiers.