Authorities are still investigating the deadly shooting of three police officers in Baton Rouge, Louisiana on Sunday morning - but they say the attack was aimed at police.

The gunman's "movements, his direction, his attention was on police officers," state police Col. Mike Edmonson said. He would not elaborate but said the shooter was definitely "seeking out" police.

"He ambushed these police officers," Edmonson told CNN. "His prey was those police officers."

Police are still trying to determine whether the gunman, Gavin Long, acted alone when he opened fire on police officers as they responded to a call of a gunman and shots being fired at a convenience store less than a mile from police headquarters.

The three officers killed were Matthew Gerald, 41, Brad Garafola, 45 and Montrell Jackson, 32 - Gerald and Garafola were white, Jackson was black. All were married and had children - Jackson had a four-month-old baby.

The shootings echoed a deadly sniper-style attack in Dallas earlier this month that left five police officers dead. In a YouTube video Long praised the killings of the Dallas officers, saying, "It's justice."

He also says: "You've got to fight back. That's the only way a bully knows to quit."

Anger over police shootings

In other online posts and videos attributed to Long he railed against police abuse of blacks. A series of deadly shootings of black men and boys by police over the past three years have roiled the country, exacerbating tensions with the police as well as racial tensions.

Few, if any, of the officers captured on videos using deadly force against suspects who were either unarmed or restrained have been found guilty of wrongdoing. Indeed, another police officer was acquitted on Monday of wrong doing in the April 2015 death of Freddie Gray in Baltimore.

Six officers were charged in connection with Gray's death. He suffered a broken neck during a violent arrest. He died in the back of a police van, unsecured, on the way to the police station.

On Monday Baltimore City Circuit Court Judge Barry Williams found Lieutenant Brian Rice not guilty.

Rice, 42, was the highest-ranking officer charged after Gray's death - facing charges of involuntary manslaughter, reckless endangerment and misconduct in office.

But Judge Williams said, "A mere error in judgment is not enough to show corruption."

Of the six officers charged none have been convicted, three have been acquitted.

Long, the gunman in Baton Rouge, was a US Marine sergeant who served in Iraq and made the dean's list in college. A native of Kansas City, Missouri, Long chose his 29th birthday to carry out the attack. He was killed in a gun battle with police.

bik/kl (AFP, AP, Reuters)