The Importance of Contacting Experienced Legal Counsel. . . Before Charges are Filed.
Prosecutors are increasingly being admonished or penalized for trying to stop or influence the testimony of potential defense witnesses. Recent media articles contain examples of the prosecution abusing its power to influence defense witnesses. Sometimes, they use their power to convince people not to testify for the defense. Other times, they resort to threats and intimidation to "convince" people not to testify for the accused. See Laurie L. Levenson, Interference with potential defense witnesses, 32 Nat'l L.J. 18 (Col. 1) (4/5/2010).
Because you do not know if your case will involve an unethical prosecutor, it is critical that you contact a criminal defense lawyer at the first hint of trouble - even if you just think you may be under investigation. It is particularly important to contact a lawyer before charges are filed. This could help preserve evidence and witnesses before the prosecution gets a hold of them. You cannot be sure that favorable witnesses will remain favorable after the prosecution has talked to them.
When you pick an attorney, it is critical to contact someone that not only has experience to find favorable evidence, but who also is plugged into a network of private investigators who know the proper and ethical steps to preserve favorable evidence. This way, if you are ever charged, you will have access to all the evidence in your favor - not only the physical evidence, but also the testimony of witnesses.