Suspect in Dallas Sniper Attacks Believed to Be Lone Gunman

The sniper who shot 12 Dallas law enforcement officers, five of them fatally, was a "mobile shooter" and was the lone gunman in the attack, the city’s mayor said Friday.

"We believe now that the city is safe, and the suspect is dead, and we can move on to healing," Mayor Mike Rawlings said Friday. Multiple law enforcement sources also told NBC News that investigators believe the gunman was the only shooter.

The gunman was identified as Micah Xavier Johnson, 25, an Army veteran who once served a tour in Afghanistan. He was killed by a bomb delivered by a police robot during an armed standoff in a parking garage after the rampage, police said.

Multiple law enforcement sources told NBC News that a small quantity of Composition 4, or C-4, was used to neutralize Johnson. C-4, also known in Britain as PE-4, is a highly malleable plastic explosive that can only be detonated with extreme heat or a shock wave from a detonator.

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Police in the immediate aftermath of the shooting said they believed two snipers opened fire on police officers. Rawlings said Friday that was incorrect.

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"This was a mobile shooter that had written manifestos on how to shoot and move, shoot and move, and he did that," Rawlings said Friday afternoon. "He did his damage, but we did damage to him as well," Rawlings said.

In the standoff with police in the parking garage, the gunman told negotiators he was upset over recent police shootings elsewhere, and "stated he wanted to kill white people, especially white officers," Dallas Police Chief David Brown said earlier Friday.

Related: Suspect 'Wanted to Kill White People, Especially White Officers'

Dallas police said Friday that detectives found bomb making materials, ballistic vests, rifles, ammunition and "a personal journal of combat tactics" in Johnson’s home. Johnson used a SKS rifle and a handgun in the attack, multiple law enforcement sources told NBC News.

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Investigators have found no ties between Johnson and any extremist groups, senior law enforcement sources said.

The shooting occurred near the end of a demonstration and march protesting recent police shootings of African-American men in Louisiana and Minnesota. Hundreds attended the event, and were sent running as gunfire erupted.

Police initially said that two snipers fired at officers, suggesting a coordinated attack. Rawlings said Friday that was due to the chaotic nature of the scene. He said some in the crowd were dressed in protective gear and carrying rifles during the event, adding to confusion about how many shooters there were.

"As we started to unravel this fishing knot, we realized the shooting came from one building at different levels by this suspect," Rawlings said.

Related: These Are the Officers Slain in Dallas

One of those people with rifles in the crowd was mistakenly identified by police as a "person of interest” in the immediate aftermath of the shooting. That man, Mark Hughes, said he was carrying an AR-15 exercising his Second Amendment rights, and tried to help others out of the area before learning through the media he was a person of interest.

Hughes turned himself in after learning he was being called a person of interest, and was later released. "I didn't understand how I became a suspect," he told MSNBC Friday. "I hadn't done anything wrong. For me to go from a person who was actually assisting — with the traffic, helping people get out of harm's way — to becoming a suspect was mind blowing."

Police are still investigating if anyone had knowledge about the attack, Rawlings said.