She was a prosecutor seeking police reform, now she says she faces a racist conspiracy

St. Louis Circuit Attorney Kim Gardner has filed a federal civil rights lawsuit against the city and the St. Louis police union that alleges a coordinated, racist conspiracy that she says is meant to drive her from office.

The complaint alleges racist and illegal efforts of Gardner’s opponents to block reforms meant to benefit minorities.

The lawsuit filed by Gardner, the first black woman elected as the chief prosecutor in St. Louis, appears to mark the first time an elected local prosecutor has brought a federal case against the police union for racially-motivated civil rights violations.

"As a reformer and black woman, I represent a clear threat to the police union and political establishment that are determined to preserve the status quo in St. Louis -- a status quo that benefits the few in power at the expense of the many," Gardner said in a statement accompanying the lawsuit.

"This lawsuit is a signal that the voice and will of the people of St. Louis, who have fought tirelessly to change a broken criminal justice system, will not be stifled."

The lawsuit cites data from the Missouri State Attorney General showing that African Americans in the state are subjected to more traffic stops and arrests (nearly twice as many) than whites in Missouri.

Several racist social media posts allegedly made by Missouri police officers are also listed in the complaint. In one, a former police division officer allegedly, "posted a photograph on Facebook of an African American police officer standing with two African American demonstrators, calling the officer 'Captain ‘Hug a Thug' and a disgrace to the uniform.'"

In another, a lieutenant is alleged to have posted, "I’m not sure what the hell is going on in our country these days. I just drove by an authentic Mexican restaurant in town; and there were white guys putting on a new roof, cutting the grass and doing landscaping."

Another Facebook post allegedly made by another Missouri officer, "offered to sell a t-shirt emblazoned with the words, 'Black Lives Splatter, because Blue lives matter.'"

The lawsuit also alleges that the Saint Louis Police Officers Association "has gone out of its way to support white officers accused of perpetrating acts of violence and excessive force against African American citizens," including the August 2014 shooting of 18-year-old Michael Brown by Missouri police officer Darren Wilson.

Additionally, the SLPOA paid the bail of Jason Stockley, a white officer who shot and killed an African American motorist named Anthony Lamar Smith in December 2011, according to the court complaint.

Gardner, who sought to launch several police reforms, including improving investigations of police misconduct in St. Louis, received threatening letters sent to her office that were "filled with racial invective," the lawsuit alleges. In the letters, Gardner was called the n-word, and also various sexist expletives, according to the lawsuit.

While Gardner’s lawsuit portrays her as a victim of a racist police force and entrenched power structure she dared to challenge, she has also faced substantive accusations of incompetence after her office’s indictment of Missouri Gov. Eric Greitens.

Gardner's former investigator, William Don Tisaby, has been charged with perjury and evidence tampering for his role in the Greitens’ investigation. Gardner is scheduled to give a deposition in the Tisaby case on Wednesday.

The police union issued a statement calling Gardner's lawsuit "frivolous and without merit."

"The union believes this is a grand distraction meant to misdirect the attention that Gardner's deposition is sure to generate," the union's statement reads.

Union officials renewed calls for Gardner, who was elected in 2016, to step down or "to be removed from office through any means available," according to the union's statement.

Her dispute with the police union reflects broader tensions in the country between police and progressive prosecutors elected as a check on the system.

Attorney General William Barr has denounced prosecutors like Gardner as "[George] Soros-backed" crusaders who represent a danger to law enforcement.

The lawsuit, filed in the Eastern District of Missouri claims to take aim at "entrenched interests" that Gardner says have tried to thwart her reforms through a "broad range of collusive conduct," including the appointment of a special prosecutor to investigate her office.

ABC News' Samara Lynn contributed to this report.