Conspiracy deniers are liberals who, in knee-jerk fashion, deny any possibility of foul play if the truth may be embarrassing to the Left in any way. They often use the epithet "conspiracy theorist" to try to shut down an investigation or even a question about a suspicious, apparently criminal act, in order to cover up the embarrassing truth.
"Lone gunman" is a favorite expression of conspiracy deniers, even though forensic evidence may show multiple gunshots from different directions. Often the Deep State is anxious to announce they have caught the "lone gunman," and that no one else was involved in a violent crime, in order to close the case as an ostensibly successful investigation.
However, when the implication of a Republican seems possible, then conspiracy deniers are the biggest conspiracy theorists of all, as when they pushed the Muellergate in order to try to embarrass President Donald Trump.