From http://www.seattleweekly.com/2011-02-23/news/ian-birk-why-he-wasn-t-charged-and-what-happens-next/
"(...)Last week, although SPD's own Firearms Review Commission deemed the shooting "unjustified" and Birk resigned, there was still anger (although not enough to provoke a riot; see above) directed at King County Prosecutor Dan Satterberg for not charging Birk. But that decision was inevitable, as Washington law protects officers like Birk from prosecution over the use of deadly force in all but the most egregious cases.

That specific state law (RCW 9A.16.040) lists 10 different ways in which a peace officer can legally kill someone—including one that says an officer can do so if he or she has "probable cause to believe that the suspect, if not apprehended, poses a threat of serious physical harm to the officer or a threat of serious physical harm to others."

(...)

But to take this point further, the law also says that "a public officer or peace officer shall not be held criminally liable for using deadly force without malice and with a good faith belief that such act is justifiable pursuant to this section." In other words, Satterberg would have had to prove that Birk didn't actually believe Williams posed a threat and that he shot him with the direct intention of committing a crime.

Proving what anyone believes is a nearly impossible legal task. Thus, it's the law itself, more than anything else, that kept Birk from being held criminally liable for Williams' death."

-jh