Martin Luther King Jr had invited the Rev James Reeb down to Selma, Mississippi - which was at the center of voting rights demonstrations at the time - to take part in a protest march to Montgomery.
But the Boston based minister never made it as he was set upon by angry white mob and beaten in a brutal attack, dying two days later from his injuries.
Three white men — Elmer Cook, William Stanley Hoggle and Namon O'Neal "Duck" Hoggle — were tried on state murder charges and acquitted by an all-white jury.But 46-years on G-Men are hoping modern techniques including DNA might finally find out who was behind the killing which shocked America at the time.
It part of an initiative by the agency to investigate unsolved murders from the civil rights era which they set up in 2007.
The lead state prosecutor for Selma, District Attorney Michael Jackson, said he has met with FBI agents at least twice about the case.
A spokesman with the agency, Chris Allen, said: "Like all cold cases, it will be difficult to prosecute because some of the suspects and witnesses have died, but we are hopeful nonetheless of finally being able to lay this case to rest."