In a letter released Wednesday, the department faulted county police procedures and pointed to warning signs that preceded the 2008 killing of immigrant Marcelo Lucero.
The department is urging Suffolk County to revamp of practices that it says have have hurt relations with Hispanics.
Federal authorities are reportedly continuing their investigation into whether Suffolk police have violated constitutional or civil rights of Hispanics.
County Executive Steve Levy said some of the recommendations are constructive but he disagrees with others.
Advocates for Hispanic immigrants said the report confirmed Suffolk's shortcomings.
Luis Valenzuela of the Long Island Immigrant Alliance said the said the climate in the county is anti-immigrant.