A thorough and competent investigation almost always makes the difference between an acquittal or a conviction. The district attorney's office and/or the attorney general's office will be using every available resource to prosecute your case. It is therefore absolutely imperative that you also have an investigator working on your side.
The investigator's job is two-fold: to gather evidence in support of your defense and to verify and validate all that has been done by the investigating police department. The latter function requires the investigator to possess enough training or experience to detect whether all proper procedures were carried out during the police investigation. For this reason, your defense attorney must retain an investigator who has experience handling the particular offense with which you were charged. For example, if you were charged with Driving Under the Influence, a former police officer who is also an expert in DUI and standardized field sobriety tests would be an ideal investigator for your case. It also beneficial to involve an investigator who previously worked in law enforcement as a police officer or detective. Such an investigator has first-hand experience interviewing witnesses, working closely with prosecutors, and identifying the "weak points" in the prosecution's case.
Throughout the investigation process, your attorney must provide adequate and thorough guidance to the investigator. However, an experienced and skilled investigator need not be micromanaged. He or she must be able to interpret pertinent statutes and policies and to otherwise work independently when conducting investigatory duties.
Qualified investigators in the Los Angeles area typically charge on an hourly basis, and virtually every case requires an investigator. If you cannot afford an investigator, you may be eligible to access public funds to hire one.