DALLAS -- 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 one final, cryptic message: the letters "RB," scribbled 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:
The carnage could have been much worse
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 plenty of supplies to make explosives. Brown said police found bomb-making materials and a journal at the shooter's home that suggested he'd been practicing detonations and appeared ready to take aim at larger targets.
It was enough, Brown said, to have "devastating effects on our city."
He '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 the death of Alston Sterling -- a black man killed by police after he was already pinned to the ground.
"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 had also 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 those 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?"
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, told The Blaze his son started delving into black history after he was honorably discharged last year. But he had no clue
"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."
Ongoing investigations in Louisiana, Minnesota
While Dallas tries to determine what led to the shootings in their city, authorities in Louisiana and Minnesota are investigating the controversial deaths of two black men at the hands of white officers.
The U.S. Department of Justice is leading a criminal investigation into Sterling's death in Baton Rouge, Louisiana Gov. John Bel Edwards said. The FBI and state police also will be involved, and a federal civil rights investigation will be conducted.
Edwards dismissed any questioning of Louisiana's decision to hand over the investigation to the Justice Department.
"We're not abdicating anything," the governor said, adding that "we're making the best decisions to make sure that the situation remains under control here in Baton Rouge and that we don't experience any more upheaval ... in our communities here in Baton Rouge and around Louisiana."
Baton Rouge police Lt. Johnny Dunham said there was body camera footage of the shooting -- even though the cameras were dislodged. The cameras continued to record, he said. A law enforcement source told CNN that the cameras fell off during the scuffle.
Investigators said they'll review multiple videos of the shooting, and they're canvassing for witnesses.
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."