You guys really have absolutely no idea how an investigation works, do you? For starters, there is a federal civil rights investigator and the FBI conducting their own inquiry as to what happened. That means, they have to identify any potential witnesses and take statements from them. After all potential evidence is gathered, a grand jury is convened to determine whether charges will be brought against the officer. The discrediting of witnesses occurs at the time of a criminal trial where either side tries to impeach the credibility of the witness, which in this case, will likely include the officer's testimony.
In that regard, if there were in fact death threats made against the police officer involved, withholding his identity at this time is more than reasonable. That doesn't mean there's a cover-up, because the identity of the officer will eventually be made known as the investigation nears its end. If you guys are going to get so emotionally riled up about this case, do some of your own research and know what you're talking about instead of making ASSumptions about everything based on your OWN prejudices about the police.