In many cases, two or more crooks commit crimes together. When you catch them, you'll generally do your best to get admissible confessions from them. Arresting multiple suspects can actually give you better chances to obtain statements.
Lawful Interception and Recording
For starters, you may leave your arrestees cuffed in the cage in the back of your police car with a concealed tape recorder running in the front seat. If you walk away out of earshot, they may scramble to come up with a story to tell, and you can get it all on tape. Because prisoners have no reasonable expectation of privacy in your police car, there's no Fourth Amendment problem with this technique. (Lanza v. New York) And since you did not engage in interrogation, there's no need for Miranda warnings and waivers before your prisoners talk to each other. (Arizona v. Mauro)
If there's no urgent necessity for immediate interrogation, you could next put them into a bugged cell to hear and record what they say between themselves about their predicament. A recording of their volunteered statements is constitutionally admissible, for the same reasons (no "search," no "interrogation").
Assuming your suspects need a little coaxing to talk about the case, the next option would be to put an undercover officer or custodial informant into the cell with one or more of them to try to stimulate conversations about their crimes ("What are you guys in for? How did the cops get you? I'm getting out tomorrow, if you want me to get rid of your stuff or anything.") Since the suspects would not be aware they are talking to a government agent, Miranda would not apply. (Illinois v. Perkins) (This technique cannot be used after the suspects are indicted or arraigned and assert their Sixth Amendment right to counsel. U.S. v. Henry)
One Against the Other
When it's time to begin interrogation, you will usually pull out the suspect who seems most likely to waive Miranda and confess. This will often be the youngest, or the one with the cleanest record, or the one who looks most scared, or the person you believe has the least amount of involvement in the crime. If this person waives and talks, selected excerpts from his recorded confession can be used to show the next suspect that the beans have already been spilled.
Even if the first suspect doesn't admit anything, you can return him to the cell with a big "Thanks for everything, Danny," right in front of his cellmate. The second suspect can then be taken out for Mirandized interrogation, and bits of information from the police car or bugged cell conversations can be used to make the suspect believe his accomplice just ratted them both out. (Frazier v. Cupp) If this one talks, his statement can then be used during renewed interrogation of the first one, for the same effect.
The Bruton Complication
One of the rights an accused person has under the Sixth Amendment is the right to confront and cross-examine the government's witnesses against him. The U.S. Supreme Court ruled in Bruton v. U.S. that this right is violated if a confessing defendant's statement is used against a non-confessing defendant at their joint trial (and assuming the confessor does not take the stand to be cross-examined by his codefendant).
Bruton and his codefendant Evans were tried for the armed robbery of a Missouri post office. Evans had confessed to agents that he and Bruton committed the crime; Bruton made no admissions. At their joint trial, the judge allowed the jury to hear the agent's testimony about the confession and instructed jurors not to consider the statement as evidence of Bruton's guilt. Both men were convicted, but the Supreme Court reversed Bruton's conviction on the ground that his Sixth Amendment right to confrontation had been violated by introduction of the confession in a joint trial. The court ruled that the jury could not realistically be expected to ignore one defendant's confession to the extent it also implicated his codefendant:
"There are some contexts in which the risk that the jury will not, or cannot, follow instructions is so great, and the consequences of failure so vital to the defendant, that the practical and human limitations of the jury system cannot be ignored. Such a context is presented here, where the powerfully incriminating hearsay statements of a codefendant are deliberately spread before the jury in a joint trial.
"We hold that, because of the substantial risk that the jury, despite instructions to the contrary, looked to the incriminating hearsay statements made by Evans in determining Bruton's guilt, admission of Evans' confession in this joint trial violated Bruton's right of cross-examination secured by the Confrontation Clause of the Sixth Amendment."
Because of the Bruton rule, officers should not assume that one suspect's full confession will be adequate proof of non-confessing suspects' guilt. To avoid Bruton error, the prosecutor in such a case must either "redact" (edit) the confession to eliminate all references to the fact that others were also involved, or "sever" the defendants for separate trials.
When you arrest two or more suspects, it's important to make every lawful effort to get confessions from each of them. Every perpetrator should admit his or her own, personal involvement in the crime. In order to make each defendant's statement admissible in a joint trial, an interrogating officer should take two statements—one in which the suspect tells everything that everyone did, and a second one in which he talks only about his own actions. Each defendant's separate personal confession can then be used against him in a joint trial, without violating any codefendant's confrontation rights.
If both suspects have waived Miranda but only one has confessed, the two can be brought together and the confessor asked to repeat his full confession, implicating the other suspect. If the other suspect does not deny the allegations made against him by the confessor, this may be considered an "adoptive admission" by the non-confessor.
An adoptive admission occurs when someone else makes a statement in a person's presence and under circumstances where it would be logical to expect the person to make a denial if the statement falsely implicated him, but he does not deny the allegations. In most jurisdictions, adoptive admissions can be introduced against both the confessing and the non-confessing codefendants in a joint trial. No Bruton error would occur in such a case, because the statement is considered to have been adopted by the non-confessor as his own confession. Obviously, a person does not need to confront and cross-examine himself about his own statement.
Local Rules May Vary
States may exclude adoptive admissions and may impose greater restrictions on eavesdropping and recording under state law than required under federal standards. Always check with local advisers to learn variations in your jurisdiction.