Another bus driver was murdered here on Wednesday, bringing to seven the number of drivers killed amid an offensive against public transportation by El Salvador's powerful gangs.The death of Elias Emilio Melendez, 21, was confirmed by a police spokesman.The enforced shutdown of public transportation continued on Wednesday and affected a number of routes in San Salvador.Since Monday, transport workers on many bus lines have not been working following gang threats and murders of several drivers.The government blames the assault on the Barrio 18 gang and said if the gang's activities continued on Wednesday, it would send the army into the streets.Gangs in central America derive much of their income from extortion rackets targeting bus operators and small businesses. Victims who refuse to pay often end up dead.