Announcement of the police investigation that found the shooting was unjustified came after prosecutors said they will not file criminal charges against Birk in the fatal shooting.
"At my direction, the Office of Professional Accountability investigation will continue forward. The completion of this investigation is not contingent on Ian Birk remaining on the force," Seattle Police Chief John Diaz said. "Reaching our own administrative conclusion is a necessary step to providing a small degree of closure to the many people affected by this tragedy over the past several months. "
Seattle Mayor Mike McGinn said he believes Birk "saw the writing on the wall."
"He could read the same Firearms Review Board report that the rest of us did," the mayor said in a written statement.
Protesters call for justice
Just hours after Birk's resignation took effect, a group of some 300 people rallied in downtown Seattle in protest of the prosecutor's decision not to file charges against the officer.
But what began as a rally at Westlake Center quickly turned into a march against police brutality.
The crowd marched to City Hall, then to the intersection of Boren and Howell where Williams was shot. They then marched to the East Precinct where they were met by a police barricade.
"You're going to hell!" one woman yelled, addressing Birk.
"We evidently got a problem," said Eric Johnson, another protestor. "They let Birk go. We've got a 25-year old law that needs changed. Cops are accountable to nobody. It's shown multiple times on the news."
The marchers blocked traffic at times, but remained non-violent and eventually dispersed peacefully.
King County Prosecutor Dan Satterberg said Birk's fatal shooting of Williams, while tragic, could not be prosecuted under state law.
"We do not and legally cannot put officers on trial for murder or send them to prison for exercising their decision to use deadly force in good faith and without malice, however tragic the outcome may be," King County Prosecutor Dan Satterberg said during a news conference on Wednesday.
"Washington law gives police officers more protection against criminal prosecution for homicide than it gives to ordinary citizens.
"In order to prosecute officer Birk we would have to prove beyond a reasonable doubt that he acted with evil intent," he said. "There is no proof of this."
Satterberg said there is no evidence to refute Birk's claim that he acted in good faith, and therefore Birk cannot be prosecuted under state law. Satterberg, though, said he found the shooting troubling.
"The officer appears to have made serious tactical errors that compounded the potential for risk to himself," Satterberg said.
"We wonder why the officer decided to stop Mr. Williams, why he drew his gun, why he did not call for backup? By his own actions officer Birk closed the gap between himself and the man with the knife."
Shortly after Satterberg made the announcement, Seattle Police Chief John Diaz said at a news conference that the department's Firearms Review Board concluded that the shooting was not justified and that Birk did not follow his training.
Birk's attorney said his client is relieved he will not face criminal charges, but will carry for the rest of his life the burden of knowing he killed a man.
"I think that if Officer Birk had an opportunity to get into a time machine and go back, things would be different," said Ted Buck. "I suspect the same would be with Mr. Williams' approach, if we could talk about that."
A fast encounter
Williams, 50, was shot to death by Birk on August 30 on the northwest corner of Boren Ave. and E. Howell St.
The shooting occurred after Birk stopped his patrol car and activated his emergency lights when he saw Williams working with the knife on a board.
Police said Birk fired after Williams, who was standing about nine feet away, after Williams refused to obey Birk's orders to drop the knife.
Audio from a camera in Birk's patrol car indicates that from the moment Birk gives his first command, Williams had seven seconds to react until the first shot was fired. The video does not show the actual shooting, however.
During an inquest hearing, Birk testified that he followed his training when he saw Williams flash a knife and take a fighting stance.
"There is no doubt whatsoever in my mind that I was facing the blade of a knife when I made the decision to fire," Birk said.
A witness testified that Williams never pulled a knife on Birk, and never lunged at the officer.
Dash cam video from Birk's patrol car captures the sights and sounds of the incident.
The inquest jury split its decision over the question about the true danger the 27-year-old police officer faced.
All eight jurors agreed unanimously that Williams, a First Nations tribal member, had a knife in his hand when Birk fired his weapon.
But four of the jurors said the blade was not open at the time of the shooting, and the four others said it was unknown. None of the eight said definitely that the blade was open.
Jurors also were divided on the question of whether Birk himself believed that Williams posed a threat of serious physical harm. Half said, "Yes," the other half said it was unknown.
Satterberg said the inquest jury's decisions were helpful, but the charging decision rests with him.
He added that Birk may face a civil lawsuit.
'This is a case that has rocked our department'
Seattle Police Deputy Chief Clark Kimerer, who chaired the Firearms Review Board that investigated the shooting, said the board was unanimous in its decision that the shooting was not justified.
Kimerer said the board found a number of problems with Birk's actions leading up to the shooting, including that he failed to properly identify himself as a police officer and did not take efforts to find cover and instead advanced toward Williams.
"Citizens cannot be expected to obey people who do not properly identify themselves as Seattle police officers," Kimerer said.
He said Birk failed to notify police dispatchers of exactly what he was dealing with and did not request backup even though there was "ample backup" nearby.
Despite Birk's testimony that he initially thought he was responding to a low-risk situation, Kimerer said Birk exited his police vehicle with his weapon drawn, which officers are trained to do only when dealing with potentially imminent threats.
"These are damning findings," Kimerer said, adding that the officials involved in the investigation "deplore and decry and seriously question the actions of Officer Birk."
Kimerer said the investigation was "heart wrenching" because what investigators found was "an outcome that could have been avoided."
"These are among the most egregious failings that I've seen," said Kimerer, who has participated in dozens of shooting reviews. "This is a case that has rocked our department."
Kimerer said Birk will remain stripped of his badge and gun and that Chief Diaz will "not engage in shortcuts that could result in a disciplinary decision being overturned."
Before a final decision is made Birk is entitled to be interviewed by the department's Office of Professional Accountability and answer their questions, Kimerer said.
Once a decision is made, Kimerer said, Birk is also legally entitled to one last chance to tell his story to Diaz.
Diaz did not take questions at the news conference but said he would be moving forward with the final decision process without delay.