Officer Darren Wilson will not face criminal charges for shooting and killing Michael Brown, according to the decision by a grand jury. It's a controversial ruling, one that seems almost certain to fracture that community and the country.

I know firsthand how difficult it is to prosecute police officers. In 2010, I was the chief federal prosecutor in Seattle. That year, there were a number of high-profile incidents involving use of force by Seattle police officers. Many were caught on video (including one that showed a gang-unit cop yelling he was going to "kick the f-ing Mexican piss" out of a prone and unarmed suspect), and the images weren't pretty.

Things reached fever pitch when Seattle police officer Ian Birk shot and killed John T. Williams, an unarmed Native American woodcarver. Williams was walking on a downtown Seattle street, tool in hand. As he crossed the street in front of a police car, the officer got out, followed Williams and ordered him to drop his knife.

Seven seconds later, when Williams failed to comply, the officer shot him multiple times. Later, the officer testified that he had felt threatened.

Like the shooting of Michael Brown, this case went to local and state authorities for review of possible criminal charges. In January 2011, a local inquest jury found that the officer was not in danger and that Williams (who had hearing impairments) did not have adequate time to drop his knife.

But a majority of jurors also found that the officer did believe Williams was a threat. They made this seemingly contradictory ruling because the state sets a very high legal burden for prosecuting police. Under state law, the prosecutor must prove that an officer acted with malice and without a good-faith belief that the shooting was justified. There was insufficient evidence to meet that standard, so the local state prosecutor determined that state charges could not be brought.

Many in the community protested the decision and called on federal authorities to act.

My office joined the U.S. Department of Justice's Civil Rights Division to conduct two investigations: a criminal civil rights probe and a separate broader look at whether the police were systematically using force in an unconstitutional way. (This is happening right now in Ferguson, Mo., too.)

After looking at the facts, we concluded that we couldn't bring criminal civil rights charges. Federal law sets a very high bar, and it essentially requires proof beyond a reasonable doubt that an officer intended to deprive a person of his civil rights. Evidence that an officer feared for his life or acted according to training could defeat such a case. It is exceedingly difficult to prove such specific motivation. On the one hand, this is OK — we want police to be able to make split-second decisions necessary to protect us. But we also want to ensure that deadly force is used only where necessary.

We spent a difficult day meeting with the victim's family, law enforcement authorities and community groups to explain the decision.

But broad and enduring change was still possible. Even where individual criminal cases cannot be brought against an officer, a system that fosters unconstitutional policing can be corrected.

We reviewed voluminous documents and data, and conducted dozens of interviews and meetings with community members and law enforcement authorities. Eventually, our other investigation concluded that the Seattle Police Department had a pattern of using unconstitutional force and found troubling evidence that it acted with racial bias.

Months of acrimony followed. We engaged police and political leaders, and consulted with national experts on all aspects of policing. This type of broad outreach is challenging but essential. Involving cops in the solution is particularly important. Opposition was stiff at times. Political leaders were bitterly divided. Even after agreement was reached and a consent decree entered, pockets of resistance remained. But under threat of litigation, the city finally agreed to a broad consent decree entered in federal court. The order required wholesale changes in how and when police used force, how they were trained and how they will be held accountable.

The process of remaking the department began. Now, every aspect of reform must be reviewed and approved by the federal judge and his appointed monitor. This ensures independence, helps insulate the process from political and budgetary pressures, and increases public trust and confidence.

New policies and training on using force, dealing with the mentally ill, and biased policing were developed. A Community Police Commission, made up of a broad cross-section of community members and police officers, was created to oversee the changes and foster positive dialogue. The parties are in the process of agreeing to and measuring outcomes. One significant benefit: There already is formalized collaboration with the mental health provider community, and all dispatchers and officers have received training on how to deal with people in crisis.

Today both the city and the department have new leaders who have embraced reforms. Years of work remain to implement the new policies and truly change the culture. But all parties — community, police, elected leaders and the Justice Department — are building the type of department the city needs and wants. These are the lasting changes that are possible in any city, including Ferguson.

Jenny Durkan was U.S. attorney in Western Washington for five years, until October 2014. She served on Attorney General Eric Holder's Advisory Committee and served on the Civil Rights Subcommittee. She wrote this article for the Washington Post.