There does not seem to be any evidence of Sergio's involvement. He does not look like the person on the neighbor's surveillance video who murdered Elizabeth Barraza. While statistically many times it turns out the boyfriend/husband had something to do with it, I think police would have followed any connection by now. If someone who Sergio knew had access to a Nissan Frontier Pro-4x truck, that would be of interest, but I think police would have looked all over Tomball, TX for that truck early on in this case.
Unfortunately, he will probably always be considered a suspect by people until the case is solved. This is due to the statistic I reference above. Sometimes people think when a case remains unsolved for a long time that police know who did it but just cannot prove it.
I think it could possibly be a neighbor, but that is an opinion. It would not surprise me if the person who murdered Elizabeth Barraza was walking around the crime scene area after the murder observing along with everyone else. They would not be able to be observe right away because in my opinion they would first have to hide the truck and then take some form of transportation back to the neighborhood.