Sadly, you certainly aren't the first person to reach out feminist.com with an injustice involving police officers. It happens not only when they are the actual assailant, but also when they are even friends with an assailant. It really is corrupt. It's certainly within the DA's jurisdiction to prosecute the cop, they are guilty regardless of supposedly be law enforcers. I would contact a group that works specifically on the issue of sexual assault, it's likely that they have worked with similar cases before and might have suggestions of how to work around it.
It really all depends on your county, and your state and other very specific details. The more local the resource, the more likely they can help you. I would start with (800) 799 SAFE.