BATON ROUGE, Louisiana - At about 8:40 a.m. on Sunday morning, some Baton Rouge police officers inside a convenience store less than a mile from police headquarters noticed a man standing behind a beauty salon wearing a mask, body armor, and dressed in all black carrying a rifle.

The man was former Marine and Kansas City native Gavin Eugene Long, it was his 29th birthday, and officials said Monday that he was there to kill cops.

Col. Mike Edmonson with the Louisiana State Police told a press conference that Long "certainly was seeking out police," adding: "His movements, his direction, his attention was on police officers."

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The attack was, according to Edmonson, an "ambush."

In the end, three police officers were killed, and three more were wounded in Baton Rouge in an assault that has only added to the heightened tensions across the country between the black community and the police.

Edmonson also confirmed that investigators have interviewed people with whom the shooter had contact with in Baton Rouge. But Edmonson wouldn't say how many or give details. He stressed that the interviews don't mean that those people were involved in the shooting and urged any others who might have had contact with or information about shooter Gavin Long to come forward.

Two of the slain officers were from the Baton Rouge Police Department: 32-year-old Montrell Jackson, who had been on the force for a decade, and 41-year-old Matthew Gerald, who had been there for less than a year.

The third fatality was Brad Garafola, 45 and a 24-year veteran of the East Baton Rouge Sheriff's Office.

Jackson, who was black, posted a message on Facebook on July 8, just three days after the death of 37-year-old Alton Sterling, a black man killed by white Baton Rouge officers after a scuffle at a convenience store.

In the message, Jackson said he was physically and emotionally tired and complained that while in uniform, he gets nasty looks. When he's out of uniform, he said, some people consider him a threat.

Jackson pleased with readers: "Please don't let hate infect your heart."

Long's online presence shows initially an author with a passion for motivating people, but his recent videos urged viewers to "fight back" against oppressors.

Long's tours of duty included Japan and Iraq. He rose to the rank of sergeant. Military records show he received several medals during his military career, including one for good conduct, and received an honorable discharge.

But just last year, he aligned himself with a movement, which federal authorities have called a "growing domestic threat," reports CBS News correspondent Dean Reynolds.

In a video posted 10 days before the shooting, Long said he was "affiliated with the spirit of justice."

"Don't affiliate me with nothing," Long said. "Yeah, I was also a Nation of Islam member, I'm not affiliated with it. ... They'll try to put you with ISIS or some other terrorist group -- no."

Long declared himself a "sovereign citizen," part of a group that believes government and law enforcement does not hold any authority.

In a FBI law enforcement bulletin in 2011, the agency called sovereign citizens "a growing domestic threat to law enforcement" and said "the FBI considers sovereign-citizen extremists as comprising a domestic terrorist movement."

Louisiana has also dealt with deadly threats to police from those affiliated with the sovereign citizen movement previously. In 2012, some sovereign citizens were among the seven people arrested in a fatal shootout with Louisiana deputies near New Orleans.

A video, which Long said he was recording from Dallas after the police killings there, condemns any peaceful protest.

"It's only fighting back or money, that's all they care about -- revenue and blood," Long said in the video.

His last tweet, posted about seven hours before the shooting, proclaimed: "Just [because] you shed your physical body doesn't mean you're dead."

Local court records show Long married in 2009 but divorced two years later. He had no known police record.

Baton Rouge has been tense since the July 5 shooting of a black man pinned to the ground by white police officers.

At a three-hour service Friday, mourners paid their respects to 37-year-old Alton Sterling, whose shooting outside a convenience store began a tumultuous week in race relations in America.

Last week, police arrested and identified three young people who they say plotted to kill Baton Rouge cops using guns stolen from a pawn shop. Law enforcement said at a conference they believe it to be a substantial and credible threat on police officers in the Baton Rouge area.