The Grand-Jury Clause of the Fifth Amendment applies only to the federal government, not the states, but even so, about half of the states choose to require a grand-jury indictment in order to prosecute a felony case.
In other states, like California, prosecutors have the discretion to decide whether to proceed by indictment or by complaint, preliminary hearing, and information. See Cal. Const. art. I, § 14; Pen. Code §§ 737-40, 889, & 949. If they proceed by presenting the case to a grand jury and obtaining an indictment, there is no preliminary hearing. See Cal. Const. art. I, § 14.1.
Most counties in California, like Orange County, have 19-member grand juries. See Pen. Code § 888.2. Los Angeles County has 23-member grand juries because its population exceeds 4,000,000. Id. A 19-member grand jury needs twelve votes to return an indictment; a 23-member jury needs fourteen. Id. § 940. From the time they are impaneled, grand jurors typically serve one-year terms.
A grand jury may investigate both civil and criminal matters, including by investigating the fiscal or administrative affairs of the county in which it operates and reporting any public corruption or misconduct it finds. Any county may impanel a second, additional grand jury for these purposes, and Los Angeles County may impanel an additional two. See Pen. Code §§ 904.6(a) & 905.8.
In criminal matters, the standard to indict is probable cause: that is, whether the evidence would lead a person of ordinary caution or prudence to believe, and conscientiously entertain, a strong suspicion of the guilt of the accused. Pen. Code § 939.8. See Cummiskey v. Superior Court (1992) 3 Cal. 4th 1018, 1027-29. That’s the same standard used at a preliminary hearing to hold a defendant to answer the charges and bind him over for trial.
Generally, the government may not present inadmissible evidence to a grand jury, including hearsay. See Pen. Code § 939.6. See also People v. Superior Court (Mouchaourab) (2000) 78 Cal. App. 4th 403, 424-25. There’s an exception to the rule against hearsay, however, for the testimony of qualified law-enforcement officers who relay the hearsay to introduce and authenticate documents, exhibits, or other physical evidence for the grand jury to consider. Pen. Code § 939.6(c). Even if the grand jury receives inadmissible evidence, moreover, its indictment will survive if it’s supported by enough admissible evidence. Id. § 939.6(b).
If the government becomes aware of exculpatory evidence, it must inform the grand jury of its existence, and it must inform the grand jurors of their duty and power to order the production of such evidence if they have reason to believe it will explain away the charge. See Pen. Code §§ 939.7 & 939.71. See also Johnson v. Superior Court (1975) 15 Cal. 3d 248; McGill v. Superior Court (2011) 195 Cal. App. 4th 1454, 1464, 1517 (adding that the government may not tread on the grand jury’s independence by attempting to dissuade or discourage it from considering such evidence).
If the government fails to do that, and its failure causes substantial prejudice to the defense, the court can dismiss any part of an indictment that was based on the evidence. Pen. Code § 939.71. See Berardi v. Superior Court (2007) 149 Cal. App. 4th 476, 490-95 (explaining “substantial prejudice” as a reasonable probability that the grand jury would not have found probable cause to indict in light of the omitted evidence).
In general, grand-jury proceedings are secret. See Pen. Code § 915. All communications with grand jurors and witnesses must be placed on the record, but all grand jurors must take an oath to preserve the secrecy of the proceedings. Id. §§ 911 & 938. This oath is also administered to other participants in the proceedings, including court reporters, language interpreters, security personnel (in the case of an inmate-witness), and qualified support persons (in the case of a child-witness). See id. §§ 939, 939.11, & 939.21.
The oath of secrecy does not apply to witnesses, however. Although witnesses are often admonished by the prosecutor or the grand jury not to disclose their testimony or other information, the admonition alone does not carry the force of law in the absence of a court order, and it may be challenged on First Amendment grounds as a prior restraint on speech, which is subject to strict scrutiny by the courts. See San Jose Mercury News, Inc. v. Criminal Grand Jury of Santa Clara County (2004) 122 Cal. App. 4th 410, 414-18.