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The investigator can testify to his investigation only. If the investigator has had special training, such as traffic schools, then you can seek to "qualify him as an expert," which is done by notifying the court you seek to qualify him as an expert and then get him on the stand and question him about his training and experience relative to accident scene investigations and establish he has sufficient training and experience and ask the court to allow him to testify as an expert. Once you qualify him as an expert, you can then ask his opinion about fault or cause of the accident.
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