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A St. Paul police officer who shot and killed a man who rushed at him with a knife in September will not face criminal charges, the Ramsey County Attorney's Office announced Thursday, calling the use of force legally justified.

Officer Steven Mattson fatally shot 31-year-old Ronald Davis on Sept. 15. Body camera footage released days later showed the deadly, seconds-long encounter that began with Davis ramming the back of Mattson's squad car.

Mattson had barely gotten out of his car when Davis was seen charging toward him with a knife and his arm raised.

"Whoa, whoa, whoa! Holy [expletive]!" Mattson exclaimed, falling to the ground briefly before standing back up. "Get the [expletive] … Get away from me!"

"Get the [expletive] up, man," Davis was heard saying as he charged again toward Mattson and picked up a flashlight the officer had dropped in the street.

Mattson warned Davis to drop the knife several times before firing two fatal shots.

The encounter lasted 13 seconds from when Mattson exited the car to when shots were fired. In the days after, St. Paul Mayor Melvin Carter and Police Chief Todd Axtell said Mattson acted appropriately.

In a 13-page memorandum, the Ramsey County Attorney's Office determined the shooting was "objectively reasonable and necessary under the circumstances of this case." Ramsey County Attorney John Choi supported the verdict in a separate e-mail memo.

"I hope that the public will recognize that peace officers are asked to respond to difficult and dangerous encounters with members of the public, and that deadly force is the only option in a circumstance like the one presented to Officer Mattson on September 15," Choi said.