The US Department of Justice (DOJ) has released a major report on the Chicago Police Department's 'pattern' of violations of civil rights and federal laws in recent decades.

"Chicago Police Department (CPD) engages in a pattern or practice of using force, including deadly force, in violation of the Fourth Amendment of the Constitution," the DOJ said in a statement.

The DOJ launched the probe into the 12,000-officer force — one of the largest law enforcement agencies in the country - after a police video showing the fatal shooting of 17-year-old Laquan McDonald was released. The footage, which shows a white police officer firing 16 shots at a black teenager, caused public outcry and protests, and led to calls for an investigation.

The video contradicted the accounts given by Officer Jason Van Dyke, who was involved in shooting, and other police officers on the scene, who stated that McDonald had had a small knife with its blade folded, and thus posed a threat to the officer's life. Van Dyke was charged with first-degree murder on the same day the video came to light.

Chicago Mayor Rahm Emanuel has initiated a few police department reforms since the investigation began, including issuing body cameras to officers on patrol.