CHICAGO (AP) — St. Louis's elected top prosecutor says the city, its police union and others violated a Civil War-era law by allegedly engaging in a racist conspiracy to force her from office and prevent her from reforming racist practices.

In a lawsuit filed Monday, Circuit Attorney Kim Gardner said the defendants violated the the Civil Rights Act of 1871, also known as the Ku Klux Klan Act. Gardner is black; the named defendants are white.

Here is a closer look at the law:

WHAT IS THE KU KLUX KLAN ACT?

The act was signed by President Ulysses S. Grant in April 1871 to help enforce the 14th Amendment, which had been ratified three years earlier.

Although the amendment defined citizenship and guaranteed due process and equal protection of the law to all, including freed slaves, vigilante groups including the Ku Klux Klan continued to threaten blacks and their white allies, undermining Reconstruction efforts.

The act was one of a series of actions designed to eliminate violence and protect the civil and political rights of 4 million freed slaves.

The U.S. Supreme Court has since said the act can be invoked in civil lawsuits against public officials, but not private citizens, said Jack Beermann, a Boston University law professor.

WHAT DOES IT DO?

The KKK Act originally authorized the president to intervene in former Confederate states where civil and political rights were being violated. It also allowed people to sue for monetary damages if their constitutional rights had been violated by someone acting under state authority.

Today, it is invoked frequently in lawsuits involving police brutality or voting act violations, though it's rarely called the Ku Klux Klan Act, Beermann said. He said Gardner's claim also is unusual because it involves a public official suing other public officials.

HAS THE ACT BEEN INVOKED IN OTHER HIGH-PROFILE CASES?

The act was cited in a 2017 lawsuit against the white supremacists who organized a rally in Charlottesville, Virginia, that turned violent.

Lawyers for 10 people who experienced violence at the massive rally of white supremacists and counter-protesters say conversations among organizers that included what kinds of weapons to carry was a conspiracy to commit violence against racial minorities. Beermann said that argument could be difficult to win because it involves only private individuals.

To win a case alleging a conspiracy, he said, the plaintiff must show that public officials were involved in a racially motivated conspiracy to deprive someone of their constitutional rights.

“Racial motivation could include trying to stop reform of racist institutions,” Beermann said.

In St. Louis, Gardner's lawsuit cited racist Facebook posts by St. Louis police officers, which led to the firing of two veteran officers. The suit also says the police union went "out of its way" to support white officers accused of violence and excessive force against blacks, including posting bond for an officer who was charged with murder. The union said it considered the lawsuit “frivolous and without merit.”