You really think the agency wouldn't have verified handwriting BEFORE releasing it to the public? Come on. AFAIK, there isn't a "suicide" investigation. It's a SSA stolen identity case.
I'm guessing that what Owutatangledweb means by "agency" is the local law enforcement people: police or sheriff's department.
So did the local LE do an investigation into her death? My guess is that because it was a prima facie case of suicide, the Ruffs were a local family with roots & connections in the community, & there is no obvious reason to suspect foul play, they ruled it a suicide within an hour or two. I'm sure they did some basic investigation just to be sure -- determine that she died in the vehicle, maybe perform a quick handwriting comparison with known samples (if any exist) & her suicide notes -- the usual due diligence. (After all, competent law enforcement officers don't like closed cases being reopened & exposing all sorts of unrelated embarrassing mistakes.)
I doubt any of the local LE even imagined that the Ruffs might actually murder her & stage it to look as if she committed suicide. After all, what would the Ruffs gain? Blake was filing for divorce, he'd likely end up with custody of the daughter, & eventually LEK would drift away from the community. They'd only consider murder as an option if they were violent & impulsive people -- & for all of their possible faults I've seen no evidence of that. Thus the local agency probably didn't spend much time on her death: it was the holidays, we know there was no autopsy, & there are always other demands on their time.
But my guess is just the opinion of a person whose knowledge of how law enforcement works is based on second-hand knowledge. Maybe they did a thorough investigation, maybe they gave the vehicle a perfunctory glance & dismissed it as a suicide so they could get back to enjoying Christmas. (It does seem odd that no one checked the house for a few weeks after LEK's death; I would imagine that performing due diligence in this case would involve investigating her home, just to confirm there was nothing that contradicted that this was suicide.)
The only way to be sure what they did in this case would be to file a FOIA request, if that can be done under Texas law. If for no other reason than to avert any distracting conspiracy theories; after all, this case is baffling enough to not need any speculation about KGB agents.