AZTEC, N.M. — Authorities said Wednesday they are close to wrapping up a months-long investigation into a school shooting that left two students dead and rocked a rural community in New Mexico.

The San Juan County Sheriff's Office said in a statement there is a handful of theories as to why the gunman opened fire inside Aztec High School on Dec. 7, killing Francisco "Paco" Fernandez and Casey Jordan Marquez shortly after classes started that morning.

But the sheriff's office said the only factual conclusion investigators have made is "that the shooter had serious issues and was hell bent on mass casualties for his own personal notoriety."

Investigators declined to comment further, saying they plan to release more information once the inquiry is complete.

The 21-year-old gunman, William Atchison, ambushed Fernandez in a bathroom and then encountered Marquez in the hallway. Both students were shot multiple times before the gunman walked the hall, firing randomly. He then killed himself as students and teachers took cover in their classrooms.

Authorities have said notes left behind by the gunman detailed his plans to gear up after making his way onto campus and then shoot up a classroom. The notes reflected frustrations with work and life in general.

The shooter did not have a criminal record, officials have said. The only contact with law enforcement was what they initially described as a generic message on an online gaming forum in 2016 in which he talked about what weapons might be used in a mass shooting.

The posting was flagged, and the FBI talked with Atchison at his home in Aztec, where he lived with his parents. At the time, he did not own any weapons other than an airsoft pellet gun. He told authorities he had no plans for an attack and just liked to troll sites online.

Aztec Police Chief Mike Heal said the FBI did not provide city police with information about Atchison's online activity prior to the shooting. Instead, he said they provided information about Atchison's brother.

The Daily Times reported that Heal answered questions about what police knew about Atchison during his monthly coffee event at City Hall on Tuesday. The topic of the discussion was school security.

Heal said Aztec police created a sketch of Atchison's brother, believing that the brother was behind the online posts.

FBI spokesman Frank Fisher declined to comment, citing the ongoing investigation.

Heal said there didn't appear to be anything out of the ordinary when two of his officers spoke casually with Atchison the night before the shooting at the gas station where he worked. The officers were there filling up their patrol cars.