A new lawsuit filed by a Latino advocacy group charges that some officers in the Suffolk County Police Department routinely targeted Latinos for traffic stops based on their ethnicity, then robbed them or gave them unjustified summonses.

The lawsuit was filed in U.S. District Court in Central Islip on Wednesday on behalf of 21 Latinos who live in Suffolk County, and alleged that the department allowed the practice to go on for a decade.

"People of color should be able to walk and drive the streets of Suffolk County without fear of being harassed and robbed by Suffolk County Police Department officers," attorney Juan Cartagena of the advocacy group LatinoJustice, which filed the lawsuit, said Thursday.

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The lawsuit comes a year after Suffolk Police Sgt. Scott Greene was charged with taking money from a Latino driver he had pulled over during a sting operation.

Greene, a named defendant in the suit, was arrested in January 2014, just after the department reached a tentative agreement with the U.S. Justice Department. The agreement ended the federal monitoring of the department, which began after the 2008 hate-crime killing of an Ecuadorean immigrant.

LatinoJustice and another advocacy group, Make the Road New York, said at a news conference Thursday in Central Islip that after the Greene arrest, "dozens" of other people came forward. The new witnesses "were too afraid, or thought it pointless, to complain about widespread police criminality," the groups said.

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Separately, LatinoJustice released a letter it had written last Friday to the U.S. attorney's office in Brooklyn asking for a Department of Justice investigation into its allegations.

The group said DOJ should approach Suffolk County the same way it approached Ferguson, Missouri, after a fatal police shooting of an unarmed black man there.

"Neither the Latino, nor the African-American, communities of Suffolk County should have to wait for an ugly, racially motivated incident before such an independent inquiry is undertaken, given the unprecedented scope of evidence of discriminatory policing already in hand and the harm it has inflicted," the group said.

A spokeswoman for the U.S. attorney's office had no comment. The Suffolk County Police Department released a statement saying it did not comment on pending litigation, but added that: "The public should know that when we receive a complaint about one of our officers' conduct, we treat it seriously and will act upon and investigate it fully."

A spokesman for District Attorney Thomas Spota released a statement saying LatinoJustice had not given prosecutors any information about robberies involving the plaintiffs.

"As we have done continuously since the beginning of this investigation, we continue to urge the victims to contact the district attorney's office," the statement said.

Most of the plaintiffs in the case were stopped and robbed while driving, but others were confronted by police while in a parked vehicle or on the street, the groups said. They asked Spota to expand his investigation beyond Greene.