Washington (CNN) The Justice Department formally closed its investigation of Ferguson Police officer Darren Wilson, declining to bring criminal charges for the killing of Michael Brown.

In a report released Wednesday, prosecutors said that "Wilson's actions do not constitute prosecutable violations" of federal civil rights law.

"There is no evidence upon which prosecutors can rely to disprove Wilson's stated subjective belief that he feared for his safety," the Justice Department report said.

The Justice Department also issued a 100-page report that it said found systemic racial discrimination by the Ferguson police and court system against African-Americans.

At a Wednesday evening press conference, Ferguson Mayor James Knowles outlined a number of reforms the city had implemented to address some of the Justice Department's concerns, and said the city "must do better" to address racism.

"We must do better not only as a city but as a state and a country. We must all work to address issues of racial disparity in all aspects of our society," he said.

Knowles also said one of the three city workers who the DOJ report identified as having sent racist emails had been fired, and the other two were under investigation.

Attorney General Eric Holder said a "highly toxic environment" existed between Ferguson police officers and the city's African-American residents before Wilson shot and killed Brown last year.

"It's not difficult to imagine how a single tragic incident set off the city of Ferguson like a powder keg," Holder said.

He pointed to the use of excessive force overwhelmingly against African-American residents, noting that only African-Americans were bit by police dogs, and said "no alternative explanation" except racial bias exists to explain it.

Holder also said Ferguson's police department violated residents' First Amendment rights to record the activities of officers, regularly conducted illegal searches and unlawfully detained citizens and competed with each other to "see who can issue the largest number of citations in a single stop."

He said the city's municipal courts and local government "relies on the police force to serve essentially as a collection agency."

In a press conference later on Wednesday, St. Louis County Prosecutor Robert McCulloch said the Justice Department decision to not press federal charges against Wilson was not surprising, given the evidence.

He also took the opportunity to slam the federal agency for leaking information to the media.

"The only pattern and practice I can talk about is the pattern and practice of the Department of Justice of leaking information to the media," he said. "[No one is saying] there haven't been instances of racial profiling and other profiling, but to suggest that somehow it's all that goes on out there in fact does a great disservice to everybody."

The report will form a basis for the Justice Department's effort to reach a court-supervised agreement to require reforms at the Ferguson police and courts. Similar agreements, known as consent decrees, are in force with multiple police agencies around the nation, including New Orleans and Cleveland.

The report provides the Justice Department's account to answer some of the most controversial issues in the case, including whether Brown was an aggressor and whether Wilson killed Brown as he was trying to give up.

The Justice Department investigation found that Brown reached into Wilson's squad car and that a struggle ensued. Prosecutors couldn't corroborate Wilson's claim that Brown reached for his gun, but couldn't find any evidence to disprove Wilson's account. Brown moved at least 180 feet away from Wilson, but then turned and moved toward the officer, prosecutors said. Several witnesses claimed that Brown had his hands up, signaling surrender, when Wilson shot him. Some gave varying accounts, and some later recanted those claims made in media interviews.

The report says: "While credible witnesses gave varying accounts of exactly what Brown was doing with his hands as he moved toward Wilson -- i.e., balling them, holding them out, or pulling up his pants -- and varying accounts of how he was moving -- i.e. 'charging,' moving in 'slow motion' or 'running' -- they all establish that Brown was moving toward Wilson when Wilson shot him."

In a separate report, the Justice Department described what it said was a "pattern and practice" of discrimination against African-Americans by the Ferguson police and municipal courts.

The department suggested 26 recommendations, including requiring the Ferguson police to providing training to ensure officers aren't using bias in policing; that officers practice community policing by getting out of their cars and getting to know their communities; and that the police focus stops, searches and ticketing on protecting the public instead of as a fundraising method for the city's coffers.

The investigation found that the Ferguson police and courts used minor traffic and other violations to raise money for the city, targeted African-American motorists for traffic infractions, and black residents disproportionately for violations such as jay walking.

The report issued on the Ferguson police's practices cites racist emails exchanged by city employees to show a culture that abetted discriminatory behavior.

"Our review of documents revealed many additional email communications that exhibited racial or ethnic bias, as well as other forms of bias. Our investigation has not revealed any indication that any officer or court clerk engaged in these communications was ever disciplined," the report says.

Nor did they ever see cases of someone asking others to refrain from sending such emails or reported as inappropriate.

A March 2010 email mocked African Americans through speech familial stereotypes, using a story involving child support. One line from the email read: "I be so glad that dis be my last child support payment! Month after month, year after year, all dose payments!"

An April 2011 email depicted President Barack Obama as a chimpanzee.

A June 2011 email described a man seeking to obtain "welfare" for his dogs because they are "mixed in color, unemployed, last, can't speak English and have no frigging clue who their Daddies are."

An October 2011 email included a photo of a bare-chested group of dancing women apparently in Africa, with the caption, "Michelle Obama's High School Reunion."

A December 2011 email included jokes that are based on offensive stereotypes about Muslims.

According to the report, each of these email exchanges involved supervisors of the Ferguson patrol and court operations.

A Justice Department official told reporters that some of the emails were sent by people still involved in helping to negotiate a settlement with the Justice Department. The Justice Department declined to identify them.

City officials informed the Justice Department that "while many FPD supervisors have their email accounts on hard drives in the police department, most patrol officers use a form of webmail that does not retain messages once they are deleted."

According to the report, members of the community recall officers using epithets in public:

In August 2014, an African-American man was having an argument when officers responded and the man was pulled out of his apartment. He responded "you don't have a reason to lock me up" officer responded "N-----, I can find something to lock you up on". Man responded 'good luck with that'. Cop then slammed man's face into a wall and when he fell said "don't pass out motherf-----, because I'm not carrying you to my car."

In July 2014, a man described walking with friends past a group of Ferguson officers who shouted racial epithets at them as they passed.