Mother of man killed by Fairview police disputes account of shooting
She waited outside. When the officers arrived, her son came to the door of their second-story apartment, at the top of 14-step staircase, holding a kitchen knife, 8 to 9 inches long. At the bottom of the stairs, officers unholstered weapons, she said.
"I said 'Put the knife down, Larry,' and they go, 'Drop the weapon! Drop the weapon,' and then they shot him three times. It happened that fast."
http://www.oregonlive.com/gresham/index.ssf/2012/01/mother_of_man_killed_by_gresha.html
(I am not so sure the 21 foot rule applies when the guy is up a flight of stairs.)
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http://www.oregonlive.com/gresham/index.ssf/2012/02/grand_jury_clears_two_fairview.html
Sgt. Bernie Meyer said the department has a "21-foot-rule" covering the shooting. Officers are taught that a knife-wielding suspect could run 21 feet before they could draw their weapons and fire.