Excerpts:

When the Russian bots jumped on the hashtag #Parklandshooting — initially created to spread news of the shooting — they quickly stoked tensions. Exploiting the issue of mental illness in the gun control debate, they propagated the notion that Nikolas Cruz, the suspected gunman, was a mentally ill “lone killer.” They also claimed that he had searched for Arabic phrases on Google before the shooting. Simultaneously, the bots started other hashtags, like #ar15, for the semiautomatic rifle used in the shooting, and #NRA.

The bots’ behavior follows a pattern, said Mr. Morgan, one of the researchers who worked with the German Marshall Fund to create Hamilton 68, the website that monitors Russian bot and fake Twitter activity. The bots target a contentious issue like race relations or guns. They stir the pot, often animating both sides and creating public doubt in institutions like the police or media. Any issue associated with extremist views is a ripe target.

The goal is to push fringe ideas into the “slightly more mainstream,” Mr. Morgan said. If well-known people retweet the bot messages or simply link to a website the bots are promoting, the messages gain an edge of legitimacy.

By Friday morning, the bots that pushed the original tweets around the Parkland shooting had moved on to the hashtag #falseflag — a term used by conspiracy theorists to refer to a secret government operation that is carried out to look like something else — with a conspiracy theory that the shooting had never happened.

By Monday, the bots had new targets: the Daytona 500 auto race in Daytona Beach, Fla., and news about William Holleeder, a man facing trial in the Netherlands for his suspected role in six gangland killings. It is unclear why.