A local detective once told us that a senior officer had said to him, "Always ask the suspect for a confession; you never know when you'll get one." Good advice, indeed.
A confession is the prosecutor's best evidence. Even if a suspect is "caught red-handed," the investigating officer should always ask for a statement. Physical evidence can often be suppressed or "explained away." It can even be impeached by attacking the motives or expertise of the officer who collected it. Witnesses can be impeached by implying that they are mistaken, or by accusing them of improper motives. A confession will often foreclose those attacks on the other evidence.
Remember to ask about the little things, too. To prove a prior conviction, the prosecutor must produce a certified copy of the judgment entry and a witness who can identify the offender from that prior case. If a suspect admits a prior conviction in his or her confession, no identification witness is needed.
Investigators should also ask where the crime occurred. In a few recent drug abuse cases, the offenders admitted drug use, which was confirmed by urinalysis. However, defense attorneys challenged the charge because there was no evidence that the crime took place within the court's jurisdiction! A quick question about the time and location of the crime will preclude that type of defense.
Statements from suspects are helpful even when they lie about their involvement. False statements about surrounding circumstances can often be disproved, leaving a suspect's credibility open to attack. Partial truths can lead to important evidence or witnesses. Never underestimate how well a lie can lead to the truth!