A district attorney at the city level in the United States is the prosecutor for a municipality or local judicial district. City lights from space. ... In countries adopting the common law adversarial system or the civil law inquisitorial system, the prosecutor is the chief legal representative of the prosecution. ...

 His/Her Duty is to prosecute suspected criminals who have broken state laws. In the case of federal crimes the US Attorney will handle the case. 

See also: U.S. District Attorney United States Attorneys represent the U.S. federal government in United States district court. ...