Special circumstances in criminal law are actions of the accused, or conditions under which a crime, particularly homicide, was committed.
Special circumstances are elements of the crime itself, and thus must be proven beyond a reasonable doubt during the guilt phase of the trial.
As such, they are formally distinct from aggravating circumstances, in that the latter are proven during the penalty phase of the trial instead.
[1] If a defendant is convicted of first-degree murder and one of 22 listed special circumstances are found to be true, the only possible penalties are life in prison without the possibility of parole or death (25 years to life if the defendant was a juvenile).
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