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, a spokeswoman for the circuit attorney's office said Thursday.

Circuit Attorney Kim Gardner did not explain why the officers were placed on the list, but said in a statement that it's crucial for her office to have confidence "in the accuracy and honesty of the oral and written reports of police officers."

"A police officer's word, and the complete veracity of that word, is fundamentally necessary to doing the job. Therefore, any break in trust must be approached with deep concern," Gardner said in the statement.

The move, first reported in the St. Louis Post-Dispatch, drew an immediate response from the state's top law enforcement official. Attorney General Josh Hawley said he was "deeply concerned," though he wasn't sure what authority his office had to intervene.

"We are exploring the potential options," Hawley said during a stop as part of his campaign for U.S. Senate. Hawley is a Republican, Gardner a Democrat.

Gardner provided the St. Louis police department with the list of officers Tuesday, her spokeswoman, Susan Ryan, said. Ryan also declined to say why the officers were on the list.

Police Chief John Hayden said the department is seeking "legal guidance on how this affects the police division."

"At this time, we are considering how best to proceed and what if any actions to take," Hayden said in a statement.

But Ryan said Gardner's office has had "in-depth conversations with the police department about our concerns."

Mayor Lyda Krewson, through her spokesman, declined comment.

Ryan said the circuit attorney's office is reviewing only open cases, not cases already adjudicated. If a police officer on the exclusion list is deemed a non-essential witness, the case will still proceed, she said.

If the officer is the witness deemed most essential to the case, the case will be evaluated further to determine if the case should be dropped.

Ryan said prosecutors have an obligation to exclude witnesses who are not deemed credible.

Jeff Roorda, business manager for the St. Louis Police Officers' Association, did not immediately respond to phone and email messages seeking comment, but told the Post-Dispatch the union was "surprised and alarmed by this unexpected action by the Circuit Attorney's office."

Gardner was elected in 2016 and has had a testy relationship with police. Though her predecessor filed the charge, it was Gardner's office that prosecuted former officer Jason Stockley, who was charged with murder in the death of a black suspect. The acquittal last September of Stockley, who is white, set off weeks of protests.

In June, Gardner dropped hundreds of traffic cases and nearly three dozen other cases brought by a Missouri state trooper, citing concerns about his conduct during traffic stops.

Hawley said he has reached out to Gardner's office for an explanation.

"I do not understand the reasons for this at all, and I can say I am deeply concerned about this," Hawley said. "Every community in this state deserves the protection of the law, the fair and equal protection of the law, and I am deeply concerned that this action will result in criminals going free, being unaccountable, and vulnerable communities not getting the protection they deserve."