BATON ROUGE, La.—A gunman fatally shot three police officers and wounded three others before being killed by police here Sunday morning, the second deadly multiple shooting to target police in 10 days.

The alleged gunman was identified as Gavin Long, a 29-year-old African-American man and former Marine sergeant, according to a person briefed on the investigation. The suspect was from Kansas City, Mo., and was affiliated with an antigovernment group, the person said.

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