The fifth and final investigation of the case was conducted by the FBI’s cold case division, with the investigation beginning in 2008 and officially closing in 2011. The purpose of this investigation, motivated by the Emmett Till Unsolved Civil Rights Crime Act of 2007, was to investigate violations of criminal Civil Rights statutes that occurred prior to 1970 and resulted in death.
After reviewing the previous investigations, the FBI ended up with a total of 10 potential witnesses. However, eight of those witnesses were deceased and two were unable to be located. With no new leads, the investigation once again concluded that Brooklyn, Belvin, Cox, and Spivey were most likely responsible for the bombing. Since all the suspects had died by this point, the case was closed.
Source: https://www.buzzfeed.com/kellymartinez/true-crime-the-unjust-murder-of-harry-and-harriette-moore