From Heather Cox Richardson in Letters from an American:
Trump drew his suspicions from a story on One America News Network, a small cable television network whose constant support for the president has drawn his attention away from the Fox News Channel, whose support is less fervent. The Buffalo man (whose name I am not using in the vain hope that he may retain some anonymity) was a retired computer programmer and longtime Catholic peace activist who is affiliated with human-rights organizations.
The story came to OANN from a right-wing fringe site famous for conspiracy theories. It was narrated on OANN by correspondent Kristian Rouz, a Russian native who has also worked for Sputnik, a news outlet controlled by the Kremlin.
When the injured man’s friends dismissed the accusations as ludicrous, OANN followed up its original story with one that showed the victim as a left-wing extremist and said he was “far from the kindly old man that many in the media are describing.” For his part, the victim’s lawyer noted that the police had made no such accusations against his client, and wrote: “We are at a loss to understand why the President of the United States would make such dark, dangerous, and untrue accusations against him.”
With the victim still in the hospital, the president’s embrace of the theory that he was a provocateur embarrassed even Republican leaders, who uniformly tried to avoid comment on the issue for reporters.