Hours after the Las Vegas massacre, Travis McKinney’s Facebook feed was hit with a scattershot of conspiracy theories. The police were lying. There were multiple shooters in the hotel, not just one. The sheriff was covering for casino owners to preserve their business.

The political rumors sprouted soon after, like digital weeds. The killer was anti-Trump, an “antifa” activist, said some; others made the opposite claim, that he was an alt-right terrorist. The two unsupported narratives ran into the usual stream of chatter, news and selfies.

“This stuff was coming in from all over my network of 300 to 400” friends and followers, said Mr. McKinney, 52, of Suffolk, Va., and some posts were from his inner circle.

But he knew there was only one shooter; a handgun instructor and defense contractor, he had been listening to the police scanner in Las Vegas with an app. “I jumped online and tried to counter some of this nonsense,” he said.