There has to be an unarguable reason as to why the FBI definitively stated that Gabby's death was homicide, and not , say, accidental, reckless negligence , some animal attack, manslaughter, etc. or even her own suicide.
And the categorical statement was swift. It didn't take months, it took a day, at most. It would be reasonable to conclude that the manner of her death was obvious, perhaps, since her murderer knew her, her face was somewhat disfigured, a sure sign that the murderer knows his/her target. Something she would not do to herself, something only someone else could do, once she was incapacitated.
It would be unlikely that the FBI would call it so firmly if there was any doubts. All eyes were upon them, no room for mistake.