MLK Had to Go

I was eavesdropping on a conversation between two African American men at the local coffee shop regarding Martin Luther King, Jr. and one observed "Once he started speaking out against the Vietnam War, he had to go." The other agreed: "he had to go." I hear this theory every now and again (see, e.g., Rage Against the Machine "You know they went after King / When he spoke out on Vietnam /He turned the power to the have-nots / And then came the shot") and wonder how prevalent it is. Given the history of racist violence in the United States, the "cover story" that he was assassinated by a white supremacist seems pretty plausible.