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“I didn’t really think he would come back. It’s more personal than that,” she said. “He wasn’t going to that second line to make national news. He was probably settling some kind of score.”

Ms. Moles, 24, said the arrest underscores the city’s crisis of violence among its young people.

“His life is over now, too,” she said of Akein Scott.

Police have not established a motive. Officials initially said three people were spotted running away from the shooting scene, but Akein Scott was the only suspect identified publicly until his brother’s arrest was announced Thursday.

Investigators launched an intense search for Akein Scott, with police Superintendent Serpas urging him to surrender at a news conference Monday and warning the teen that “we know more about you than you think we know.” At one point, SWAT team members and U.S. marshals served a search warrant at one location but did not find Akein Scott.

Police offered a $10,000 reward in the case, and investigators received several tips after images from the surveillance camera were released.

The video released Monday showed a crowd gathered for the parade suddenly scattering in all directions, with some falling to the ground. They appear to be running from a man in a white T-shirt and dark pants who turns and runs out of the picture.

As many as 400 people had come out for the event. Officers were interspersed with the marchers, which is routine for such events. The crime scene was about less than two miles from the heart of the city’s French Quarter.

Two children were among those wounded.

The mass shooting showed again how far the city has to go to shake a persistent culture of violence that belies New Orleans’ festive image.

Gun violence has flared at two other city celebrations this year. Five people were wounded in a drive-by shooting in January after a Martin Luther King Jr. Day parade, and four were wounded in a shooting after an argument in the French Quarter in the days leading up to Mardi Gras. Two teens were arrested in connection with the MLK Day shootings; three men were arrested and charged in the Mardi Gras shootings.

• Associated Press writer Michael Kunzelman contributed to this article.