The Federal government may intercede in a state case when there a larger criminal enterprise involved, when it is a specific federal crime- like bank robbery, counterfeiting or child kidnapping. They may also be working with the state in investigating a case. Most times it is discretionary with the federal authorities. Without more information, it is difficult to tell if you might be in danger of your case becoming a "federal" case. The systems are different. The rules of evidence differ in some respects and certainly the penalty/incarceration system is much different.
If the Federal Government wants a case, it can take it and there is nothing the State can do to stop it. Generally speaking, there is usually some sort of agreement or understanding that this is the way the case should be handled. If the Feds prosecute a crime, the State cannot (but the opposite is not true).
There are myriad of reason the Feds take a case and I cannot otherwise answer your question. It could have been a Federal investigation. It could be that the penalties at the Federal level are greater. It could be that the Feds would like to get someone higher up the food chain. It could be that the Feds only can prosecute everyone together because of jurisdictional reasons. I could go on ...
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