Man Arrested in Brazen Sidewalk Killing in St. Louis That Was Captured on Video

A 23-year-old man has been charged with first-degree murder in connection with a brazen sidewalk killing of a man in St. Louis on Monday, the police said, an episode that a witness captured on video that was widely shared on social media.

The suspect, Deshawn Thomas, was arrested as he tried to enter a nearby library a few hours after he had fired one shot at the head of a man who was sitting on a curb in downtown St. Louis, Detective Marco Christlieb of the St. Louis Police Department said in a brief interview on Tuesday night.

The shooting, which occurred around 10 a.m., shocked many people who saw the roughly 45-second video clip of the encounter online.

In that video, which Detective Christlieb confirmed shows the shooting, a man in yellow points a gun at a man sitting on a curb in the 700 block of North Tucker Boulevard.

The man appears to load the gun before pointing it at the head of the other and firing.

The police said that Mr. Thomas and the victim had been fighting that morning at a nearby Shell gas station before the shooting.

Mr. Thomas ran off after the shooting, and the police caught up with him about 2:40 p.m., the authorities said. The police also recovered a firearm and a shell casing.

The killing came a week after the St. Louis circuit attorney, Kim Gardner, a Democrat, faced criticism after a teenage volleyball player lost both of her legs in a vehicle crash that the authorities said had been caused by a man who was awaiting trial on robbery charges, The Associated Press reported.

Ms. Gardner, whose office did not immediately respond to an email seeking comment late Tuesday night, said earlier that her office had tried several times to revoke the man’s bond, adding that the attorney general in Missouri, Andrew Bailey, a Republican who has been critical of her, was engaging in a “political stunt,” The A.P. reported.

On Tuesday, Senator Josh Hawley, Republican of Missouri, shared on Twitter the video showing Monday’s fatal shooting and used it to cast blame on Ms. Gardner.