Christian Taylor, 19, was shot multiple times during an early-morning incident at a car dealership by a rookie officer who was nearing the end of 16 weeks of field training, Johnson said.
Johnson told reporters that a security company called 911 early on Friday after Taylor allegedly drove his Jeep through the front window of the dealership showroom. The company monitored Taylor on surveillance cameras and reported a possible burglary in progress. At one point, Taylor allegedly kicked out the windshield of one of the cars on the lot.
When officers arrived, Taylor was inside. They yelled at him from the other side of the glass to get on the ground but he ran away and tried to open a locked glass door, Johnson said.
Brad Miller, 49, and his training officer, a 19-year veteran, went inside to arrest Taylor. There was a confrontation in which Miller fired four times and the other officer used a Taser, the chief said.
Where the gunshots came in the sequence of events is unclear, Johnson said, and Miller has yet to be interviewed by investigators. Miller was not injured, but it is standard procedure to wait a few days before questioning an officer involved in a deadly shooting, the chief said.
"The facts available today do not answer all questions or alleviate all concerns," Johnson said.
He told the media that surveillance video, police radio traffic and 911 calls will be released in the next seven to 10 days, after all officers involved have talked to investigators.
"If this was not justified or authorized under the law there will be consequences," Johnson said. Taylor was black and Miller is white, and the deadly shooting came almost one year to the day after the death of Michael Brown led to protests nationwide and discussions about race and police shootings.
Arlington police have invited the Dallas field office of the FBI to participate in the investigation and review its process and findings, Johnson said.
'Unarmed...and you shoot to kill?'
Taylor played defensive back at Angelo State University, according to the team website.
Last year's roster listed him as a 5-foot-9, 180-pound freshman.
Miller, who graduated from the police academy in March, is on routine administrative leave as authorities investigate the shooting, Arlington police said.
Taylor's father said while it appears his son did wrong, he shouldn't have had to die.
"What he'd done, ain't no way right," Adrian Taylor told CNN affiliate KTVT.
"But to shoot an unarmed man? You're a police officer, you're trained to take down men with your hands. You have your Tasers, you have your clubs, whatever there is. Unarmed, a 19-year-old -- and you shoot to kill?"
Christian Taylor had "no real problems," his father said. He said he often gave a helping hand to the homeless.
"A good dude, man," the father told the station. "We'd be going over here to church and he'd pull over and give a homeless guy money, shoes if he needed and he'd have to go back home and get some more shoes because he gave his away. He was like that."
The father didn't know what could have prompted his son to allegedly drive his SUV into the dealership.
"You know, it could have been too much drinking, he could have been wrong place at the wrong time, he could have gotten something and he didn't know what he was getting," the father told the affiliate. "I don't know."
No body cam, no shooting video
Miller wasn't wearing a body camera because the department doesn't yet use them, police said. The police department "is in the process of implementing a pilot program for body worn camera use," authorities said in a statement.
Miller had no previous police experience prior to joining the Arlington force in September 2014. He has been in field training and working under the supervision of a police training officer since graduating from the academy, authorities said.
He has no disciplinary history or commendations, the department said.
A school mourns
Angelo State University director of communications Becky Brackin said the school had no details on the incident, "but our condolences go out to his family and friends," Brackin said.
The football team acknowledged Taylor's death on its Twitter page.
"Our thoughts and prayers are with the family of Christian Taylor. Your presence will be missed, but not forgotten," the Angelo State University Ram football team tweeted.