Where do I need to report bankruptcy fraud

Failure to disclose assets in a bankruptcy petition can be prosecuted criminally as fraud, but the decision whether to do so is well within the discretion of the US Attorney. Not every failure to disclose will be worthy of criminal prosecution. The role of the Chapter 13 Trustee and US Trustee is to refer cases where their investigations reveal the possibility of knowing non-disclosure. The trustees also have powers to sanction non-criminally under the Bankruptcy Code through such actions as non-dischargability actions and dismissal. If it is clear that the US Attorney is declining to prosecute, contacting the Chapter 13 Trustee about non-criminal sanctions might still be helpful. Be aware that what looks crystal clear and of highest priority to you, may not be to others.
Best wishes for an outcome you can accept, and please remember to designate a best answer.

If this is that important to you, you may have to hire a lawyer to intervene in the BK action to get some formal attention. Popping off email accusations to a trustee is not likely to get the result you seek.

If the FBI investigated and the US attorney chose not to prosecute because the amount of fraud is too low for them to bother with, that is it. The US attorney does not have to prosecute every crime they come across and if they chose not to proceed, then they have prosecutorial discretion and, unfortunately, there is likely nothing further that can be done. They obviously took you allegations seriously enough to have the FBI investigate but the FBI did not find a big enough crime to warrant spending relatively scarce resources pursuing the issue further. The US attorney is who would make the call, not the UST nor the case trustee, and he/she has made it.

You can't force a government agency to take action. The fact that the FBI was nice enough to give you a courtesy call to let you know the outcome of the investigation is surprising. The U.S. attorney has discretion to not prosecute. The chapter 13 trustee has no jurisdiction over fraud, they would just refer it to the U.S. Trustee. At this point, if you want to take this further, you will need to hire a lawyer and determine if you have standing and grounds to havethe BK dismissed.