What is the burden of proving an extrajudicial confession?

prove the corpus delicti, or the body of the crime itselfi.e., the fact of injury, loss, or harm, and the existence of a criminal agency as its cause. In California, it has traditionally been held, the prosecution cannot satisfy this burden by relying exclusively upon the extrajudicial statements, confessions, or admissions of the defendant." (People v. Alvarez, supra, 27 Cal.4th at pp. 1168-1169.) "The independent proof may be circumstantial and need not be beyond a reasonable doubt, but is sufficient if it permits an inference of criminal conduct, even if a noncriminal explanation is also plausible." (Id. at p. 1171.) Once this independent proof is submitted, the defendant's extrajudicial statementsin this case, that he bought the gun on the streets and thought it was probably stolenmay be considered to strengthen the case. (Ibid.)