The reason so many Black people believe that the FBI assassinated MLK, is because they know white US history.
They know that COINTELPRO assassinated smaller Black Messiahs for doing things like providing free lunch and clinics, and getting white steel workers to take down confederate flags. And they know that the FBI considered MLK to be the biggest Black Messiah.
US high schools teach MLK's "I have a dream" speech, but not what the deputy director of the FBI said after seeing that speech...