Though sometimes a prosecutors office will use a grand jury to fail to indict intentionally, like when the accused is a police officer they work with. It's a good way to launder away a prosecution that the DA doesn't want to pursue for political reasons.
I've banged on about this before, but every state needs to adopt a law where allegations of illegal behavior by cops are always handled by a Special Prosecutor, never the local DA. The incentives are too fucked for a DA to ever be trusted to handle it properly.
The state could contract a criminal defense lawyer to prosecute, they probably already have antagonistic relationship with police.