Criminal defense investigators impeach the credibility of a witness by discrediting their testimony through the systematic collection of evidence that reveals bias, inconsistencies, or character flaws. Impeachment is a core goal of the defense investigation, designed to ensure that the jury or court can accurately weigh the truthfulness of a witness’s claims.
According to the sources, investigators use the following methods to impeach witnesses:
1. Identifying Bias and Prejudice
Investigators look for evidence that a witness has a personal interest in the outcome of the case or a specific motive to be untruthful.
- Personal Bias: This includes uncovering racial bias, as seen in high-profile cases where investigators found witnesses to expose a person’s prejudice.
- Interests in Conviction: During trial or depositions, the defense attempts to discover if a witness has any bias, prejudice, or personal interest that would benefit from the defendant being convicted.
- Informant Motives: Investigators evaluate “paid informants” by determining their true motives—whether they are talking for money, a reward, revenge, or personal favors—as these motives directly impact their reliability.
2. Uncovering Factual Inconsistencies
A primary tactic for impeachment is proving that a witness’s story contradicts the physical evidence or their own previous statements.
- Case Review and Discovery: Investigators perform a comprehensive review of police reports and documents to find factual inconsistencies or “serious flaws” in witness accounts provided by the state.
- Locking in Testimony: Investigators take signed or recorded statements from witnesses to preserve their recollection. If the witness changes their story at trial, this written record is used to impeach them as “untruthful”.
- Conflict with Evidence: By reconstructing the crime scene or establishing timelines, investigators can prove that a witness’s version of events is physically impossible from their claimed vantage point.
3. Background Investigations
Investigators conduct deep-dive background checks to find “skeletons” or documented patterns of behavior that undermine a witness’s reliability.
- Criminal History: A witness may be discredited by showing previous convictions of crimes or a documented reputation for untruthfulness.
- Misrepresentations: Investigators may find official documents that prove a witness has misrepresented facts to the court in the past, which can be used to destroy their current credibility.
- Fitness Issues: In civil or custody cases, background checks may document extensive criminal histories or drug use among “accusers,” rendering their testimony regarding a safe environment less credible.
4. Behavioral and Forensic Analysis
- Evaluating Demeanor: During interviews, investigators analyze “mannerisms and emotional state.” Evasiveness, hesitation, or an unwillingness to discuss certain topics may signal that the person is concealing information or lying.
- Nonverbal Leakage: Cues such as a higher pitch of voice, slower speech rates, pupil dilation, or an absence of hand movements can indicate insincerity and deception.
- Challenging “Voodoo Science”: Investigators work with experts to prove that specific claims made by state witnesses are based on flawed forensics. For example, an expert might show that a blood draw gave a false positive, effectively impeaching the witness who claimed the defendant was intoxicated.
5. Surveillance to Refute Claims
In domestic or injury cases, surveillance is a powerful impeachment tool. For example, if a “claimant” alleges they have a back injury, investigators may document them engaging in heavy physical activity, like building a wooden deck or carrying full boxes. This video evidence is then used in court to prove the witness’s claims are fraudulent.