Proof & Defenses in Criminal Cases

August 18, 2018

How do prosecutors try to prove guilt? How do defendants try to disprove it? What kind of information is the prosecution and defense entitled to, and how do they introduce it at trial? This section answers these questions and others, taking an in-depth look at the way criminal trials take shape.


Discovery: What and When the Prosecution Must Disclose


When was the alleged crime committed?  

Discovery is the process through which defendants find out about the prosecution’s case. For example, through standard discovery procedure, they can: get copies of the arresting officers’ reports and statements made by prosecution witnesses, and examine evidence that the prosecution proposes to introduce at trial.  Traditionally, the prosecutor wasn’t entitled to information about a defendant’s case. But in recent years, discovery has become more of a two-way street. Just as defendants can discover information from prosecutors, so too can prosecutors examine certain evidence in the hands of defendants.

Modern Discovery Policy


Pretrial disclosure of information through discovery can foster settlement and enhance the fairness of trials.

Can prosecutors spring evidence on defendants like they do on TV?


No. In the past, prosecutors could guard evidence from defendants with the same fervor toddlers show in protecting toy trucks and dolls from their siblings. Defendants couldn’t force prosecutors to hand over witness statements or even reveal the names of their witnesses. Now the view that advance disclosure will promote fairer trials has taken hold—if defense attorneys know ahead of time what to expect, they can better defend their clients.

Surprise evidence may produce fine drama, but it leads to poor justice. Unlike prosecutors, defendants can’t call on police agencies to help them investigate and respond to evidence they find out about for the first time at trial. Thus, every jurisdiction (each state and the federal government) has discovery rules requiring prosecutors to disclose evidence to defendants prior to trial.

Are discovery rules really intended to help defendants at trial?  


Not exclusively. Sure, advance disclosure promotes fairer trial outcomes, but it also promotes case settlement, which saves judicial time and resources. If a guilty defendant finds out before trial that the prosecution has a particularly strong case, that defendant will be more likely to plead guilty and save the government the hassle of trying the case. Discovery is likely a significant reason why at least 90% of criminal cases settle before trial.
Issues regarding settlement aside, discovery is intended to help defendants in the sense that prosecutors must hand over certain information that's helpful to the defense.

Does discovery mean that the prosecution has to reveal its case strategy?


No. Discovery rules generally distinguish between raw information like names of witnesses, police reports, and drug or alcohol test results, and attorney theories and strategies. The latter is called “work product.” Prosecutors don’t have to turn over their work product to defendants—otherwise, it just wouldn’t be fair. Lawyers would be incentivized to hide their work or do less of it



Vy Tummin is charged with assault and battery on a police officer. Vy claims that she reacted in self-defense to the police officer’s use of illegal force. The prosecutor plans to show a videotape of the incident to the jury. The prosecutor also has prepared a file memorandum as a self-reminder about what portions of the tape to emphasize during the trial and why those portions are especially significant. Vy’s lawyer demands to see the videotape and all the prosecutor’s trial memoranda. Discovery rules allow Vy's lawyer to see the videotape. But the prosecutor won’t have to turn over the memorandum. The memo is the prosecutor’s work product because it contains strategic analysis.

Is there a particular period of time prior to trial when the defense is supposed to engage in discovery?


Not really. Prosecutors can’t disclose all discovery on the eve of trial, but on the other hand, they don’t have to divulge it all way ahead of time. Discovery can unfold gradually. For example, a defendant’s attorney might receive a copy of the police report at the first court appearance, but might not receive a prosecution expert’s written analysis of blood evidence until shortly before trial.


The government has a duty to preserve certain types of evidence it collects during criminal investigations and prosecutions. This duty exists in order to protect a defendant’s rights to due process and a fair trial under the Sixth and 14th  Amendments to the U.S. Constitution. The duty relates to the requirement that the government disclose evidence it will use against the defendant at trial, as well as any evidence that is favorable to the defendant. The duty to preserve evidence begins once any state agency or actor has gathered and taken possession of evidence as part of a criminal investigation.


What Kind of Evidence Must Be Preserved?

Law enforcement doesn’t need to preserve all evidence it collects. The duty to preserve extends only to evidence that might be expected to play a significant role in the suspect’s defense: “material” and “exculpatory” evidence. Material evidence is important evidence that’s directly relevant to an issue in the defendant’s case. Exculpatory evidence is evidence favorable to the defendant in that it clears or tends to clear him of guilt.


Alibi evidence

Exactly what evidence is material and exculpatory depends upon the circumstances of the case. But alibi evidence is virtually always material and exculpatory; it includes witness statements that place the defendant somewhere other than the scene of the crime and forensic evidence (like DNA) that tends to show that the defendant couldn’t have committed the crime.


Other kinds of evidence

In many cases, law enforcement doesn’t have obvious alibi evidence, but it does have other kinds of material and exculpatory evidence. An example is evidence that impeaches the prosecution's witnesses or otherwise weakens its case. Such evidence may include:

  • Crime scene evidence.  Most jurisdictions have local and state rules about the collection and preservation of evidence at the crime scene, such as the murder weapon, blood samples, and photographs of the scene.

  • Tape recordings and videotapes.  The police usually aren’t required to tape record statements of the defendant or witnesses. However, if they do make audio or video recordings of statements, they have to preserve them.

  • Investigative notes.  Most states require officers and investigators to preserve investigative notes only if they made them during an interrogation of the defendant. (But if those notes contain exculpatory information, that information must be conveyed to the defense.)

  • Emergency call recordings.  Law enforcement typically must preserve and disclose 911 recordings (or transcripts of the calls) to the defendant.

Who Has to Preserve Evidence?


While the police typically collect most evidence used in a criminal case, they aren’t the only government actors who have to preserve evidence. The duty also extends to:

  • Local, county, and state investigative agencies.  These include not just detectives and investigators, but also administrative, clerical, and forensic staff.

  • Prosecutors.  The attorneys prosecuting the case have an obvious duty to preserve evidence, as do people acting under prosecutorial authority (for example, investigators or experts retained by the prosecutor’s office).

  • The Attorney General.  In most states and in most cases, the duty to preserve evidence remains even after a defendant has been convicted. Therefore, the duty applies to a state’s Attorney General’s office (which typically handles appeals and post-conviction matters).

Private people exempted

The duty to preserve evidence doesn’t extend to private people or agencies unless they have a formal relationship with a law enforcement agency. Thus, if a private DNA lab is hired by the prosecutor to examine blood samples collected from a crime scene, that lab must protect and preserve the evidence and the results of any testing it conducts on the evidence.


What Happens If Evidence Is Lost or Destroyed?


Defendants have the burden of proving that the state violated its duty to preserve evidence, and that that violation compromised the rights to due process and a fair trial. Courts won’t do anything about an alleged violation unless the defendant proves that the evidence in question was material and potentially exculpatory and that the government acted in bad faith. In practice, it’s very difficult to prove these points.


Proving materiality

In order to prove that the evidence was material, the defendant must establish that

  • law enforcement officers had reason to believe the evidence was exculpatory before they destroyed it, and

  • the evidence can’t be replaced by other reasonably available evidence.

Courts can sometimes infer materiality from law enforcement’s actions.  For example, the fact that the state normally preserves the type of evidence that it destroyed in the defendant’s case may show that the evidence was material. Similarly, the government testing, using, or intending to test or use the evidence provides a strong indication that it recognized the importance of that evidence.


Proving bad faith

Proving bad faith is tough. It’s not enough that government actors were careless or negligent with the evidence—the defendant must show willful, deceitful, or malicious intent. The government failing to follow standard procedures when it lost or destroyed evidence can support an inference of bad faith. An example of bad faith is an officer throwing away a fingerprint sample showing that someone other than the defendant committed the crime.




There are several possible remedies for defendants who learn during trial that the state violated the duty to preserve evidence. They can ask the court to suppress related evidence, exclude or limit testimony about the missing evidence, or dismiss the case. If the missing evidence doesn’t surface until after a conviction, overturning the conviction and obtaining a new trial on appeal are possible



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