One of the essential things that must be proven in any criminal prosecution is the identity of the perpetrator. ID can be shown with three kinds of evidence: physical ("trace") evidence, the suspect's own statements, and identification by witnesses. Any of these varieties of proof can be unreliable, so the Supreme Court has set standards to determine, for each case, whether it is fair to admit certain evidence, including eyewitness ID. "Reliability is the linchpin in the admissibility of identification testimony." (Manson v. Brathwaite.)
A pretrial identification procedure is considered too unreliable if it is "so impermissibly suggestive as to give rise to a very substantial likelihood of misidentification." (Neil v. Biggers.) To be sure that you don't cause admissibility problems, you have to be careful not to suggest to the witness in any way that a particular person should be identified.
In most cases, pretrial ID will be made either at a field show-up where the suspect is being detained, or from a photo display shown to the witness, or at a lineup conducted in the jail or police station. Here are some basic rules and precautions for each procedure.
In some cases, a suspect is detained near a recently committed crime, and it's important to quickly confirm or eliminate him as the perpetrator. A victim or another witness who can make an ID may be brought to the place of detention. Or in cases where the victim is physically unable to move, the suspect may be transported for a show-up ID. The need to make a prompt determination of whether you have the right person will generally permit this procedure.
For example, in Stovall v. Denno, a couple had been attacked by an intruder who stabbed the man to death and severely wounded the woman. A suspect was soon apprehended, and officers took him to the hospital, where doctors did not know how long the woman would survive. She made an ID and the Supreme Court upheld admission of this evidence at trial, saying that "immediate hospital confrontation was imperative."
When using this procedure, be sure to document in your report the reasons why it was important to obtain a prompt ID and any steps you took to ensure a fair opportunity for accurate ID. To reduce the suggestive nature of a single-person show-up, it's best not to tell the witness that you think you've caught the perpetrator, or that you've recovered property or incriminating evidence from the suspect.
On the other hand, there's no need to discourage an ID with some lengthy admonition listing all the risks of misidentification; it may be sufficient to say, "We have someone we want you to look at. Please take a careful look and tell us if you recognize him."
If you don't have the suspect in custody (or even if you do), you can legitimately seek to obtain an ID by showing pictures to the witness. Many officers use a "six pack" photo display of mug shots or other available pictures of the suspect and five other individuals. There's no magic number, however, and, if handled correctly, even a single-photo ID is permissible. (Manson v. Brathwaite.) A similar admonition to the witness before showing photos is appropriate: "We want you to take a careful look at some pictures of a few people and tell us whether you recognize any of them."
When constructing a photo spread, it's best to use individuals who match the general description of the perpetrator. Try to use photos of people of the same sex, race, coloring, build, features, and approximate age. All photos should be in color, if possible, or else all black and white; all should be similar in size; and it's best to cover booking numbers and other identifying information. Unless the original display can be kept, a photocopy should be maintained in the case file to illustrate the fairness of the composition, in case of court challenges later.
Most of the same considerations of fairness apply to live lineups. Try to use similar people, don't do anything to call special attention to your suspect, and give a simple admonition to the witness that doesn't necessarily suggest the perpetrator is in the line, but simply asks the witness to let you know if he or she recognizes anyone.