A "charge" like from an open police investigation usually can be held over your head without filing court charges until the statute of limitations runs, and that can be years. The police may have their case against you locked in tight, but the prosecutor has instructed them to widen it and also try to build a case against other individuals related to your offense. They may even be delaying charging you so they can get a warrant to search your house or someplace else to build a stronger case against you.
There even may already be one or more people charged in the case and the prosecutor wants to finish up their cases and make a deal with them to testify against you later. He has to do that, because otherwise he cannot compel them to testify against you if it would incriminate themselves too. After their case is done, the self-incrimination protection evaporates, so they can be used against you. If you have already been indicted or charged, then there likely is a speedy trial law in your state that gives you the right to make the prosecutor go forward within from 30 - 180 days or you walk.