The Department of Justice in the US state of California has launched an investigation into two police agencies in the state which are known as the deadliest agencies in the entire country.

The investigation began on Thursday into Kern County Sheriff's Office and the Bakersfield Police Department with officials trying to examine excessive use of force and other serious misconduct by the two departments.

"These investigations will objectively, impartially, and thoroughly examine whether the Kern County Sheriff's Office or the Bakersfield Police Department engaged in a pattern or practice of excessive force or other civil rights violations," Attorney General Kamala Harris said in a statement.

"We look forward to working collaboratively with the sheriff's office and the police department, as well as with the community, to address any civil rights violations or other issues that we may find during these investigations."

This comes after a series of complaints and community groups as well as media reports have shown that the agencies’ officers killed more people than in any other county in the US in 2015.

According to the Guardian, law enforcement officers killed 14 people in 2015 in Kern County with a population of just below 875,000. By comparison, only 10 people were killed by officers in the five boroughs of New York City, whose population is almost 10 times bigger and has around 23 times as many officers.

The move was welcomed by the American Civil Liberties Union, which described it as "an important first step toward much-needed improvements in accountability and use of force in Kern County."

"More people die in police custody there per capita than any other county in the nation, and both the Kern County Sheriff's Office and the Bakersfield Police Department have unenviable records of high profile use of force incidents that often result in fatalities," the group said in a statement.

Meanwhile, the two agencies said Thursday that they will cooperate with the probe.

"We will be cooperating with the investigation fully,” said Ray Pruitt, spokesman for the Kern County Sheriff's Office.