The Los Angeles Police Protective League, which represents the department’s roughly 9,500 rank-and-file officers, warned its members in a letter this month, telling them it could lead to invasions of privacy and misuse of the information. “The privacy issues here are very real,” said union President Paul Weber. “Who is to say where the samples will be stored and who will be able to access them? There is nothing more private than DNA.” LAPD officials defend the practice as a seldom-used but important tool. After an officer uses serious force on a suspect, investigators must sometimes test blood, saliva and other genetic material found at the scene to determine whose it is and what occurred during the incident, they say.