The police can be of great service to prosecutors when it comes to finding witnesses to a crime and to following the evidence to a suspect. However, sometimes the defense is going to realize that there seems to be key evidence missing or witnesses that have not come forward: the defense attorneys are going to find a basis for reasonable doubt.

However, in order to know that their assumptions about their client's innocence are correct, they will need to conduct a criminal defense investigation.

A private investigator serves an important role in a criminal defense investigation. In this case, that role is to find witnesses and evidence that will establish reasonable doubt – that will show the jury that there is reason to believe that the defendant is not actually responsible for committing the crime.

When an attorney uses a private investigator in a criminal defense investigation, that investigator will take the time to understand the charges and the laws that relate to the crime. Once there's an understanding of the case in question, the private investigator will go over all of the materials that the defense team has received from the prosecutor.

During the course of the criminal defense investigation, the private investigator will go through routine reports from the police, everyday paperwork as well as copies of evidence, photographs, phone messages and witness statements related to the case. The goal of this is to determine whether or not there are any inconsistencies from one witness to the next or between the conclusions drawn and the evidence.

In addition, during a criminal defense investigation, a private investigator may re-visit the crime scene to see if there was anything that had been overlooked. He or she may also interview witnesses to see if their stories have changed or to verify that they do not have anything else that motivated their statements against the defendant.

Inconsistencies and ulterior motives that a private investigator discovers during the course of a criminal defense investigation may be able to be used as a part of the client's defense. Similarly, if during the course of the criminal defense investigation a private investigator discovers that there are other witnesses who had not come forward or evidence that was not considered previously, he or she can then look into these developments further.

In some cases, that may mean that a search is conducted to find these other witnesses. In other cases, it may mean conducting interviews or doing background searches. In others, there may be other parts of the criminal defense investigation that are assigned to a private investigator – tasks that he or she is uniquely qualified for and that will keep the defense team free to focus on the legal proceedings and other cases that they have.

During the course of a criminal defense investigation, a private investigator can help to ensure that an innocent client will not be found guilty – all without taking away from a defense attorney's busy schedule. In other words, bringing a private investigator into a criminal defense investigation, defense attorneys are able to focus on the court system while a private investigator conducts the investigation.