Gavin Eugene Long, the gunman who shot dead three law enforcement officials and injured three others in Baton Rouge Sunday, had pledged allegiance to an African-American offshoot of the anti-government Sovereign Citizen Movement last year, documents revealed. The 29-year-old black man was shot dead by police after the attack.

Long, a former U.S. Marine sergeant, filed legal papers in a Missouri county affiliating himself with the Washitaw Nation, an African-American group whose members view the federal government as illegitimate, Reuters reported. In documents filed in May 2015, Long wanted to change his name from Gavin Eugene Long to Cosmo Ausar Setepenra -- an indigenous identity, according to Jackson County, Missouri, public records. However, the process was never completed by Long, court officials said.

In an interview, Frederix Joe Washington, a senior member of the Washitaw Nation, denied that Long was associated with the group, Reuters reported.

"We know nothing about this man," Washington reportedly said. "We don’t give cards out, IDs out, licenses or passports. None of this has been given out by us."

Long's relationship with the Washitaw group is under probe, a U.S. counter-terrorism official said.

On Sunday, reports surfaced that Long had gone on an online rant against the excessive use of force on black people by white police officials. In one video posted online, Long also defended the Dallas shooter who killed five police officers on July 7.

Long was in Dallas in the wake of the police shootings during a Black Lives Matter rally. He rented a car in Kansas City. Missouri -- his hometown -- and drove to Baton Rouge to carry out the attacks on his 29th birthday, authorities reportedly said.

Authorities said Monday that Long targeted police to assassinate them, while bypassing civilians he encountered.

"There is no doubt whatsoever that these officers were intentionally targeted and assassinated," Louisiana State Police Superintendent Colonel Mike Edmonson said, during a news conference Monday. "It was a calculated act against those who work to protect this community every single day."

Baton Rouge Police Chief Cal Dabadie said: "I have no doubt he was headed to our headquarters, and he was going to take more lives."

Police are investigating the attack carried out Sunday by Long and are trying to determine whether it was racially motivated. Two of the officials killed were white, and a third, Montrell Jackson, 32, was black.