Former Marine Gavin Eugene Long carried out Sunday's shooting against police officers in Baton Rouge on his 29th birthday.

Long of Kansas City, Missouri, died Sunday in a shootout with police. Officers said he was wearing a mask and body armor, and was covered in all black.

Long's online presence shows 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. 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.

Long posted his thoughts in a series of YouTube videos.

"I thought my own thoughts, I made my own decisions, I'm the one who gotta listen to the judgment," Long said in one video.

Long described himself as a life coach, nutritionist and personal trainer. He wrote three books.

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."

Last May, Long filed paperwork in Jackson County, Missouri, which changed his name to "Cosmo." Law enforcement sources tell CBS News that Long used the pseudonym of Cosmo Setepenra online.

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."

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.

Sunday's shooting is a blow to a community still reeling from the death of Alton Sterling. Baton Rouge police fatally shot Sterling earlier this month, sparking nationwide protests. This latest tragedy is also a devastating loss for police departments all over the country, more than a week after Dallas lost five of its own officers targeted by a gunman.