Federal officials brought their war on medical marijuana dispensaries to Los Angeles, raiding several shops and issuing warning letters to dozens more. Officials at the U.S. attorney's office said it was the first large-scale federal action taken against cannabis shops in the city, and said more will probably follow. "We couldn't do all of L.A. at once," said Thom Mrozek, a spokesman for the office. "There's just too many stores." The crackdown adds a dramatic element to the already tense fight over the fate of medical marijuana dispensaries in the city.