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Story highlights

Authorities aren't sure why the Dallas gunman twice wrote "RB" with his own blood after killing officers

Report: The gunman's mother says he had wanted to be a cop and became "a hermit" after Afghanistan

(CNN) —  

Shortly after he gunned down five officers in Dallas, Micah Xavier Johnson sang and laughed at police. He threatened more deaths with explosives. And he left a final, cryptic message: the letters “RB,” scribbled twice on a wall with his own blood.

Were these acts inspired solely by the misplaced angst of a deranged gunman? Or was he radicalized by online groups calling for violence against police?

Here’s the latest on the investigation into the deadliest day for U.S. police since 9/11:

Explosive materials found in Dallas shooter’s home

During his hours-long standoff with police, Johnson said he was ready to kill more cops with bombs, Dallas Police Chief David Brown told CNN.

“We had negotiated with him for about two hours, and he just basically lied to us – playing games, laughing at us, singing, asking how many (police officers) did he get and that he wanted to kill some more and that there were bombs there,” Brown said.

A search of the gunman’s home revealed he had supplies to make explosives. Brown said police found bomb-making materials and a journal that suggested Johnson had been practicing detonations and appeared ready to take aim at larger targets.

It was enough, Brown said, to have “devastating effects on our city.”

Johnson ‘liked’ a group that called for violence against police

A day before Johnson opened fire on police, the African American Defense League called for action after Sterling’s death.

“The Pig has shot and killed Alton Sterling in Baton Rouge, Louisiana!” the group’s Facebook post said. “You and I know what we must do and I don’t mean marching, making a lot of noise, or attending conventions. We must “Rally The Troops!” It is time to visit Louisiana and hold a barbeque. The highlight of our occasion will be to sprinkle Pigs Blood!”

The group also issued and then quickly deleted another post Thursday, “calling on the gangs across the nation! Attack everything in blue…”

Johnson “liked” the group’s page. He also had visited the websites of the Nation of Islam and the Black Riders Liberation Party – which the Southern Poverty Law Center considers to be hate groups.

Tom Fuentes, a former associate director with the FBI, said the messages and the groups behind them should be treated the same way the federal government investigates ISIS.

“It’s no different than the ISIS propaganda that goes out,” Fuentes said. “And the question for law enforcement is, where do you draw the line between free speech and something else? If a message is espousing someone to take action, even if they inspire one guy to strike out, isn’t that enough?”

Hundreds of statements, mountains of footage

Dallas detectives are reviewing more than 300 statements from witnesses and officers from the scene, Brown said.

Police have 170 hours of body camera footage to download and analyze, the chief said.

But many questions remain, including the meaning of the letters “RB.” The killer wrote those letters twice on a wall – using his own blood – before he was killed by a police bomb, Brown said.

The sniper had a cache of weapons on him

Johnson was wearing a bulletproof vest and had three weapons on his body, a law enforcement source familiar with the investigation said.

Those weapons included a Glock 19 Gen4 pistol, a Fraser .25-caliber handgun and an Izhmash Saiga 5.45-caliber semiautomatic, assault-style rifle.

A second law enforcement official said it appears all the weapons were purchased legally, and that some were bought online.

Gunman’s parents: He wanted to be a police officer

The killings of five officers came as a horrific shock to Johnson’s parents – especially because the Army veteran had wanted to be an officer when he was younger, his mother said.

“He loved his country,” Delphine Johnson said in an interview with The Blaze. “He wanted to protect his country.”

But his demeanor and attitude changed drastically after his six-year military service, which included seven months in Afghanistan, Johnson’s mother told The Blaze. She said he morphed from a gregarious extrovert to a “hermit.”

The gunman’s father, James Johnson, said his son started delving into black history after he was honorably discharged last year. But he said he had no clue what his son would do.

“I don’t know what to say to anybody to make anything better. I didn’t see it coming,” James Johnson told The Blaze. “I love my son with all my heart. I hate what he did.”

Louisiana DA recuses himself from Alton Sterling case.

The Dallas investigation comes as authorities in Louisiana probe the controversial death of Sterling, the black man fatally shot after he was pinned down by two Baton Rouge police officers.

The U.S. Department of Justice is leading a criminal investigation, Louisiana Gov. John Bel Edwards said. The FBI and state police also will be involved, and a federal civil rights investigation will be conducted.

Was Alton Sterling killing justified? Depends what officers were thinking

Any consideration of state charges would come after the results of the federal investigation, East Baton Rouge District Attorney Hillar C. Moore said.

Moore recused himself and his office from the case on Monday, citing a possible conflict of interest.

“It is my determination as District Attorney that given the history of a long and close working relationship with the parents of one of the officers involved in this shooting, there would always be questions of my partiality,” Moore said.

He said the state attorney general could accept the case himself, appoint another district attorney or choose an independent prosecutor.

Answers wanted in Philando Castile’s death

In Minnesota, relatives of Philando Castile are trying to understand why an officer killed the school nutrition services supervisor during a traffic stop. Castile’s fiancée broadcast the aftermath of the shooting on Facebook Live.

“He let the officer know that he had a firearm and he was reaching for his wallet and the officer just shot him in his arm,” Diamond Reynolds said in her Facebook broadcast. “… You shot four bullets into him, sir. He was just getting his license and registration, sir.”

St. Anthony police don’t have body cameras, office manager Kim Brazil said. The investigation is ongoing.

“We will release the information as we learn it,” Sgt. Jon Mangseth said, “and we will address concerns as we are faced with them.”

Tom Kelly, a lawyer for Jeronimo Yanez – the officer involved in the fatal shooting – told CNN that Castile matched the description of a suspect involved in an armed robbery. Kelly said that suspect description “aroused in Officer Yanez a reasonable suspicion to take further investigative steps leading to the stop of the vehicle.”

“The driver of the vehicle largely matched the description of an armed robbery incident that happened in the vicinity of this stop, several days prior to this stop which aroused in Officer Yanez a reasonable suspicion to take further investigative steps leading to the stop of the vehicle,” Kelly said.

CNN’s Drew Griffin, David Fitzpatrick, Curt Devine, Catherine E. Shoichet, Jake Tapper, Scott Glover, Dave Alsup, Joshua Berlinger, Linh Tran and Mary Kay Mallonee contributed to this report.