Texas officials say that a suspect who was killed after a standoff with police was the lone gunman in an attack that left five police officers dead and seven wounded in Dallas on Thursday night, the deadliest day for law enforcement since the 9/11 terror attacks.

Police identified the gunman as 25-year-old Micah Johnson and said he was an Army veteran with no criminal history. He completed a tour in Afghanistan, serving in the Army from March 2009 to April 2015, according to the Associated Press.

“Others have identified him as a loner,” police said in a statement.

Dallas Police Chief David Brown said Johnson—who told police he was not affiliated with any groups—was killed in a standoff with law enforcement in a parking garage during the early hours of Friday morning.

Brown said Johnson told police he was upset with police shootings and wanted to kill white officers. The Dallas shooting began during a protest over police brutality, following the deaths of two black men by police officers in separate incidents this week.

“The suspect said he was upset about Black Lives Matter,” Brown said during a press conference Friday morning. “He said he was upset about the recent police shootings. The suspect said he was upset at white people. The suspect stated he wanted to kill white people, especially white officers.”

Johnson was killed by a police bomb robot after the lengthy standoff. “We saw no other option but to use our bomb robot and place a device on its extension for it to detonate where the suspect was,” Brown said. “Other options would have exposed our officers to grave danger. The suspect is deceased as a result of detonating the bomb.”

Dallas Mayor Mike Rawlings said Johnson had threatened to use bombs. At a press conference on Friday night, Rawlings said officials gave Johnson “plenty of options” for surrender before they used the robotic explosive.

“As it concerns the cowardice, it seems as if the sole suspect now has received his justice, and what will be important is to ensure that there are no other potential co-conspirators with this particular assailant,” Texas Gov. Greg Abbott said at the same press conference.

While Brown initially said that three suspects, including a woman, had been taken into custody by Dallas police, state and federal officials said on Friday evening that it now appears Johnson acted alone.

“As we’ve started to unravel this fishing knot, we realized that the shooting came from one building at different levels from this suspect,” Rawlings said.

Police said that during a search of Johnson’s home, detectives found bomb making materials, ballistic vests, rifles, ammunition, and a personal journal of combat tactics, which they are still reviewing.

In addition to the 12 officers, two civilians were injured in the shooting, which President Barack Obama called a “vicious, calculated and despicable attack.”

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Write to Tessa Berenson at tessa.berenson@time.com and Katie Reilly at Katie.Reilly@time.com.