TWTR Twitter Inc

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.

Authorities wouldn't immediately link the attack to the nation's recent police shootings of black men, including one in Baton Rouge, or the July 7 killing of five police officers in Dallas.

Baton Rouge has been roiled ever since police shot to death 37-year-old Alton Sterling nearly two weeks ago outside a convenience store. Mr. Sterling was black; the two officers who tried to arrest him are white. Police said officers were responding to a call regarding an armed man and said Mr. Sterling was armed.

Sunday's shooting occurred about 8:40 a.m. at a shopping center not far from Baton Rouge police headquarters, said Col. Michael Edmonson, superintendent of the Louisiana State Police. Baton Rouge officers at the scene observed a man carrying a rifle and dressed all in black standing behind a beauty-supply store, he said. Police believe the alleged gunman Long lured the officers there to ambush them, a person briefed on the investigation said.

At 8:42 a.m., police received reports of shots fired, Col. Edmonson said. Two minutes later, they received reports of officers down at the scene. A minute later, reports arrived of more shots being fired. At about 8:48 a.m., emergency responders began to arrive.

Col. Edmonson said officers "shot and killed" the suspect, who was believed to be the sole person to fire on the officers.

Police ruled out as suspects two individuals who were detained earlier Sunday across the Mississippi River from Baton Rouge in the aftermath of the shooting, believing Long acted alone, according to the person briefed on the investigation. They think the city is no longer under threat from this particular plot, the person added.

Long is thought to have traveled to Baton Rouge specifically to carry out an attack after Mr. Sterling's shooting, according to the person.

Law-enforcement officials think Long was in a rental car, surveying a police station near the shopping center where he ultimately ambushed the officers, the person said.

A website that tracks internet domains says that a person identified as Gavin Long, from Kansas City, registered a website in April called convoswithcosmo.club. A man speaking on "Convos with Cosmo" internet audio postings described his life story as one similar in many ways to the biographical information known about Long, including identical information about their service in the military, his rank, where the individual was stationed, and when he was deployed to Iraq.

"Convos With Cosmo" also had an active Twitter account and posted videos online, often complaining about the police and the treatment of African-Americans. The last tweet sent by "Convos With Cosmo" was chilling: "Just bc you wake up every morning doesn't mean that you're living. And just bc you shed your physical body doesn't mean that you're dead."

Three of the wounded officers died from their injuries, Col. Edmonson said—two from the Baton Rouge Police Department and one from the East Baton Rouge Sheriff's Office. The three included Matthew Gerald, a 41-year-old with just under a year of service, Montrell Jackson, 32, who authorities said had 10 years of service and Brad Garafola, a 45-year-old deputy with the East Baton Rouge Sheriff's Office.

In addition, three other officers—two from the sheriff's office and one from the police department—were injured. They included a 41-year old who was "in critical condition fighting for his life as we speak," said Sid Gautreaux, the East Baton Rouge sheriff. He added that each of the officers from his department was married with families.

Mr. Gautreaux delivered emotional remarks asking for healing and prayers for the community, state and nation.

"With God's help, we will get through this. To me, this is not so much about gun control as it is about what's in men's hearts," Mr. Gautreaux said. "And until we come together as a nation, as a people, to heal as a people—if we don't do that and this madness continues, we will surely perish as a people."

In brief remarks from the White House, President Barack Obama said authorities had yet to determine the suspect's motive, and he urged Americans to "temper" their words. "We as a nation have to be loud and clear that nothing justifies violence against law enforcement," he said.

Noting that the Republican and Democratic presidential conventions were imminent, he said, "We don't need inflammatory rhetoric, careless accusations to score political points. We need to temper our words and open our hearts."

The gunman, Long, attended the University of Alabama for one semester in spring 2012, where he was enrolled as a general-business major and made the dean's list, a spokesman said. Campus police don't have any records of interactions with him during his time at the university, the spokesman said.

Long served in the U.S. Marine Corps for five years as a data network specialist, including a six-month deployment to Iraq in June 2008, according to his military service record. He left in 2010 with the rank of sergeant, having spent time at stations in Okinawa, Japan and California. The Marine Corps declined to comment on the nature of his discharge from the service.

The shooting comes after the country has been trying to come to grips with several violent incidents.

Police-community relations in Baton Rouge have been especially tense since the killing of Mr. Sterling was captured on cellphone video and circulated widely on social media.

The next day, another black man, 32-year-old Philando Castile, was killed by a police officer in Falcon Heights, Minn., a suburb of Minneapolis and St. Paul. His girlfriend livestreamed the aftermath of his death on Facebook.

The Minnesota Bureau of Criminal Apprehension has said the two officers involved were placed on standard administrative leave while it investigated.

Later in the same week, five police officers were killed in a shooting in Dallas during a march protesting police brutality, which was sparked by the incidents in Louisiana and Minnesota. Police said the shooter, 25-year-old Micah Xavier Johnson, an Army veteran, had planned a wider assault on the city.

At the Sunday news conference, Mr. Edmonson, the state police superintendent, said the investigation involved multiple agencies and many moving parts. He sought to reassure the people of Baton Rouge and around the state "that we are doing everything humanly possible to make sure everyone is protected."

Louisiana Gov. John Bel Edwards said healing would take "constructive dialogue." He added, "There simply is no place for more violence. That doesn't help anyone. That doesn't further the conversation. It doesn't address any injustice perceived or real, it is just an injustice in and of itself. And we are not going to tolerate more hate and more violence tearing apart the communities and families of Louisiana."

Last week, Baton Rouge police said they had arrested three people—a 20-year-old man and two teenagers—alleging they had broken into a pawnshop and stolen eight guns. One of the men caught told law-enforcement officials during questioning that he and three others were going to get bullets to shoot police, authorities said. Later, a fourth person was arrested.

Yogita Patel, Damian Paletta and Pervaiz Shallwani contributed to this article.

Write to Jennifer Levitz at jennifer.levitz@wsj.com, Alejandro Lazo at alejandro.lazo@wsj.com and Arian Campo-Flores at arian.campo-flores@wsj.com

(END) Dow Jones Newswires

July 18, 2016 08:05 ET (12:05 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.