Hung jury. They weren't exonerated. The only option on the table regarding the death was Second-Degree Murder.
Manslaughter, and Negligent Homicide were not options here. The definition of Second-Degree Murder for New Mexico is: "Unless he is acting upon sufficient provocation, upon a sudden quarrel or in the heat of passion, a person who kills another human being without lawful justification or excuse commits murder in the second degree if in performing the acts which cause the death he knows that such acts create a strong probability of death or great bodily harm to that individual or another."
I agree with Cen-Tejas above. Because not guilty or second-degree murder were the only choices, a conviction for murder was a bridge to far for most of the jurors. A charge of manslaughter might have brought a conviction.