No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.See, by investigating murder charges Holder would be putting the officer's life in jeopardy again.
That said, the burden of proof in those cases is even higher than what the Grand Jury was dealing with. So, this statement is just to appease his race-hustler posse. Just like the Zimmerman investigation they promised.
No adjudication took place because there was just a hearing, not a trial finding facts.