We have no way of knowing if your brother is a CI, or why the police let him go in September but you are putting him in danger (from the police or other criminals) by publishing details in this very public forum. Perhaps the police wanted to follow your brother and arrest others with whom he associates. Your brother is in serious trouble. He needs a lawyer and he needs his family members and friends to stop talking in public about his case.
If a local police investigation crosses paths with a federal investigation local charges may be dropped. Then the feds pick the case up later after they have surveiled their suspects and obtained additional evidence. Maybe he was a CI or maybe he was just used by the feds to further their investigation without even knowing it. Take attorney Goldstein's advice and do not discuss this further.
This comment is being left for informational purposes only and based solely on the information provided in the question. This comment does NOT create an attorney - client relationship. An attorney should be contact directly and given the full details of the specific circumstances in order to be best able to assist in the issues at hand.
Your guess is as good as anyone else's.
DISCLAIMER The answer given above by the lawyer serves for educational purposes only and provides general information and a basic understanding of the applicable law. Take notice that the answer above does not create an attorney-client relationship as this website is not intended to provide anyone a specific legal advice. Anyone using the site expressly consents that there is no attorney client privilege between any person and any attorney responding. Further take notice that the site should not be used as a crude substitute for any professional and competent legal advice by a licensed professional attorney in the applicable jurisdiction. The attorney above attempted to provide competent professional opinion, however, the law and its applications may change frequently and vary greatly from other U.S. jurisdictions and locales. Therefore, any information and opinions stated above are general in nature, and may not apply to specific factual or legal circumstances related to one's current legal issues. Contact an experienced lawyer admitted to practice in your State under an attorney-client privilege to further receive a comprehensive legal before making an educated decision about your particular legal issue. Respectfully, Attorney Alexander Ivakhnenko, Chicago, Illinois
C.I. means Confidential Informant. Enough said. Speculation is easy, verification is difficult.
Please do NOT use this answer/response to say or do anything regarding your situation. This answer/response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. BEFORE you say or do anything consult with an experienced Federal and/or state criminal defense attorney in your jurisdiction who will listen to you and your concerns.