Ferguson riots
Demonstrators protest the shooting death of teenager Michael Brown in Ferguson in 2014. Getty

The US Department of Justice is suing the city of Ferguson, Missouri, after officials reneged on a deal to launch specific police reforms designed to protect black lives. The federal lawsuit alleges a pattern and practice of discriminatory and unconstitutional police conduct in Ferguson.

"The residents of Ferguson have suffered the deprivation of their constitutional rights, the rights guaranteed to all Americans, for decades. They have waited decades for justice," said Attorney General Loretta Lynch. "They should not be forced to wait any longer."

She vowed to "aggressively prosecute this case — and we intend to prevail." Lynch said officials had no choice but to file the lawsuit after the Ferguson City Council voted to change the terms of a deal Ferguson and federal officials had hashed out over months, National Public Radio reported.

There was no immediate response to the lawsuit from city officials. But they stressed that the City Council vote wasn't an outright rejection of the Justice Department deal, but an effort to change some of the terms because of costs.

Lynch called the consent decree deal "cost effective," but she also said: "There is no price for constitutional policing."

Ferguson authorities had agreed to the reforms to the city's police force and court system following a 2015 Justice Department investigation found that both the police and courts had discriminated against African-Americans. Officers targeted minorities disproportionately for traffic stops and the use of force, and African-Americans were more often handed jail sentences, investigators found.

"These violations were not only egregious, they were routine. They were encouraged by the city in the interest of raising revenue," Lynch said . "They were driven, at least in part, by racial bias and they occurred disproportionately against African American residents of Ferguson."

The federal investigation was launched after an unarmed black teenager Michael Brown was shot dead by police in 2014. The killing sparked weeks of riots in the streets of Ferguson.

Lynch said the City Council knew it was courting trouble when it deliberately rejected the agreement approved by Ferguson's own negotiators. "The city was well aware that by deciding not to accept it, they were choosing litigation," she said. "This was their choice."