Officials say that a fatal ambush of Dallas police officers during an otherwise peaceful protest march was the act of one man. Authorities say that Micah Johnson, 25, carried out a sniper attack by himself, killing five officers and wounding seven others plus two civilians in downtown Dallas on Thursday night.

Johnson was killed during an hours-long standoff with the police in a parking garage. In what seems to be the first such use, the Dallas Police Department sent in a robot with a bomb and detonated the device; Police Chief David Brown said, "Other options would have exposed the officers to grave danger."

According to Brown, Johnson told officers that he was upset by the police killings of black individuals and that he "planned to injure and kill as many officers as they could."

Dallas Mayor Mike Rawlings said," This was a mobile shooter who had written manifestos on how to shoot and move, shoot and move, and that's what he did... As we've started to unravel this fishing knot, we've come to realize this shooting came from one building at different levels."

Homeland Secretary Jeh Johnson emphasized in a NYC press conference, "At this time, there appears to have been one gunman with no known links to, or inspiration from, any international terrorist organization."

The gunman, Micah Johnson, lived in Mesquite, a suburb of Dallas. The NY Times reports, "He was a military veteran who had served in Afghanistan, and he kept an arsenal in his home that included bomb-making materials"— Dallas police say they "found bomb-making materials, ballistic vests, rifles, ammunition, and a personal journal of combat tactics" in the home.

Officials believe he was planning an attack for a while; CBS News reports, "Law enforcement will check where and when he bought his guns, vest, and bomb-making material to determine how long he has been planning an attack. The source said the police shootings this week were the trigger for Johnson to commit the massacre, and the march was the opportunity."

Chief Brown called it a "well-planned, well thought-out, evil strategy."

Johnson was in the Army Reserve from 2009 to 2015, and the Washington Post notes, "Johnson, who had no criminal history, deployed to Afghanistan with the U.S. Army from November 2013 through July 2014... Army records show that Johnson, whose home was listed as Mesquite, Tex., had served with an engineering brigade before he was sent to Afghanistan. He did not have a combat job and was listed as a carpentry and masonry specialist."

He left the reserve in what the Times calls "a cloud of sexual harassment charges made by a fellow soldier who sought an order of protection against him and said he needed mental health counseling."

Social media postings by Johnson expressed dismay at the recent deaths of blacks by police—"Why do so many whites (not all) enjoy killing and participating in the death of innocent beings"—and support to black power groups. From the Times:

On his Facebook page, Mr. Johnson “liked” two groups that seem to shed light on his ideological views. One was the New Black Panther Party, which was founded in Dallas, according to the Southern Poverty Law Center. The Anti-Defamation League calls it “the largest organized anti-Semitic and racist black militant group in the United States.”

The other group was the African American Defense League, which was formed in 2014 by a man named Mauricelm-lei Millere.

“Millere is known for calling for violence against police specifically, on a regular basis,” said Oren Segal, the director of the Anti-Defamation League’s Center on Extremism. “Usually after a high profile police-related shooting he takes to social media to encourage violence against police.”

After the killing of Laquan McDonald in Chicago, Mr. Millere called for “death to every blue, bastard, hypocrites, killer pig across the nation,” according to Mr. Segal.

The Daily Beast reports

, "Johnson formerly attended the 'self-defense and personal protection[' gym Academy of Combat Warrior Arts in Richardson and Fort Worth, Texas, gym owner and CEO Justin Everman told The Daily Beast. The gym’s Twitter account says it provides “'eality based training for today’s Urban environment.'"

Army Lt. Col. Michael Waltz, a former Special Forces officer and White House aide, told Reuters that based on video of the shooter taken during the ambush suggested that Johnson was “not only trained, but well trained... The way he raises and lowers the rifle to his shoulder, apparently firing two rounds at a time, with discipline in his movements and use of cover, is what makes it seem as though he has military training."

Johnson's movements turned the protest march into a chaotic scene. At 9 p.m., an officer said on the radio, "We’ve got a guy with a long rifle. We don’t know where the hell he’s at."