(CNN) -- The man paces back and forth on a St. Louis street, yelling "Shoot me," as two armed police officers approach him.

Less than 20 seconds after their arrival, the officers open fire, killing him.

As new cell phone video emerged Wednesday of the fatal confrontation that claimed the life of Kajieme Powell, a 25-year-old African-American man, St. Louis Police Chief Sam Dotson tried to explain the officers' action in an appearance on CNN.

The incident took place Tuesday, less than four miles from the suburb of Ferguson where, just days earlier, a police officer shot and killed Michael Brown, an unarmed black teen -- and touched off violent protests.

Dotson said Powell was suspected of walking out of a convenience store with two energy drinks and a package of pastries that he didn't pay for.

Police said Powell had a knife that he pulled out, although that is unclear from the video.

"An edge weapon, a knife, within 21 feet is a lethal weapon," Dotson told CNN. "The officer just did, what I think you or I would do -- they protected their life in that situation."

'Drop the knife'

The video is graphic. CNN has chosen to freeze the footage right before Powell is shot, but the audio continues.

Powell, dressed in khaki pants and a blue sweatshirt, is standing on the side of a street when a police car pulls up on the sidewalk near him.

He takes a few steps back as officers get out of the police cruiser and approach him, armed with guns.

They yell at Powell, repeatedly ordering him to get his hand out of his pocket.

Powell walks towards them, yelling, "Shoot me! Shoot me! Shoot me now."

He takes his hand out of his pocket and appears to be holding something in his right hand, but it's unclear from the video what the object is.

Authorities say Powell was brandishing a knife.

One of the officers says, "Drop the knife." Powell looks back over his shoulder for an instant, then steps over a low concrete wall, off the sidewalk, moving closer to the officers as he is ordered again, "Drop the knife."

Nine shots are heard. (In the unedited video, the man drops to the ground). Onlookers gasp.

The man filming the incident exclaims, "Oh my God."

Questions raised

Dotson said Powell was "acting erratically, walking back and forth up and down the street."

Asked why the officers didn't use alternate weapons, such as a stun gun, to subdue the man, Dotson responded by saying such defenses can be unreliable.

"So you've got an individual armed with a knife, who's moving towards you, not listening to any verbal commands, continues, says 'Shoot me now, kill me now.' Tasers aren't 100%. If that Taser misses, that subject continues on, and hurts an officer," he said.

CNN's Chris Cuomo said the officers' actions raise several questions.

"There was no raising, there was no lunge, there was no charge," he said. "They open fire, both officers open fire."

"The question now becomes, why did they have to do this?"

The two officers involved have been placed on administrative duty, per department policy.

CNN's Dana Ford and Jason Hanna contributed to this report.