WASHINGTON (CNN) -- New reports about four police officers in Fort Lauderdale exchanging racially offensive text messages and a video that showed President Barack Obama with gold teeth could prove to be the latest test for how the Justice Department polices the nation's local law enforcement agencies.

In the past five years the Justice Department has opened 20 investigations into local police departments -- more than twice as many investigations than were opened in the previous five years, according to the department.

A Justice Department spokesperson says the majority of law enforcement agencies the department investigates agree to work with the department to correct patterns of officer misconduct -- but when an agency refuses to correct the problems the Justice Department files suit against the agency.

Right now the Justice Department has suits pending against local law enforcement agencies in four jurisdictions -- Maricopa, Arizona; Alamance, North Carolina; Colorado City, Arizona; and Meridian, Alabama.

In the case of the Fort Lauderdale officers -- Jason Holding, James Wells and Christopher Sousa were fired and the fourth officer, Alex Alvarez, resigned.

The officers used racially derogatory terms to refer to people they encountered while on duty and talked about "killing n******," according to investigative documents.

An official with the Justice Department's Civil Rights Division tells CNN, "The department is monitoring the local investigation."

Late Wednesday afternoon Broward County Public Defender Howard Finkelstein -- who defends low-income residents in Ft. Lauderdale -- sent a letter to the Justice Department calling for a civil rights investigation into the Fort Lauderdale Police Department.

Finkelstein wrote, "I do not believe the Fort Lauderdale Police Department conducted a full investigation. Its investigation was intentionally limited in order to protect the department."

Finkelstein said he asked the Justice Department to investigate the same police department seven months ago and nothing happened.

In Wednesday's letter he wrote, "I wrote to your department on August 29, 2014 requesting an investigation into egregious discriminatory and race-based police practices utilized by the Fort Lauderdale Police Department. I received no response."

The public defender also said his office is now reviewing all 52 pending cases that are still open and involve any of the four police officers -- as well as all 126 closed cases that involved any of the four officers from the last two years.

"We are looking for cases in which the defendants were black males and where the stop is sketchy or the defendant says the officer called them bad names or assaulted them," Finkelstein told CNN.

He said the fact that the four officers "not only felt comfortable enough to put their racist comments in writing, but to also make a movie about it, tells me there is a racist subculture in that department." Finkelstein says he believes the problem goes much deeper than those four officers.

"We're not shocked by those officers' racist attitudes, we're just shocked they were stupid enough to make a movie about it," said Finkelstein. "Stupid knows no bounds within the Fort Lauderdale Police Department."

Fort Lauderdale Police Department for reaction to the public defender's letter to the Justice Department.

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