ST. LOUIS (AP) — The top prosecutor in St. Louis has established what she calls an “exclusion list” of 28 city police officers and is reviewing all open cases in which those officers were essential witnesses.

The action by Circuit Attorney Kim Gardner, first reported in the St. Louis Post-Dispatch and confirmed Thursday by Gardner’s spokeswoman, drew an immediate response from the state’s top law enforcement official. Missouri Attorney General Josh Hawley says he is “deeply concerned,” though he isn’t sure what authority his office has to intervene.

Gardner provided the police department with the list of officers. Her office has not said why the officers were placed on the list but Gardner says in a statement that “any break in trust must be approached with deep concern.”

Police Chief John Hayden says the department is seeking “legal guidance.”