According to California Penal Code 135, the willful destruction or concealment of evidence that will be used in a trial or investigation is illegal. This can apply to major criminal cases, but also civil cases that require the documentation of evidence. However, this crime only applies if the individual knew that the item they were getting rid of was being used in a trial; the act had to have been willful. If the individual knew the evidence was going to be used, but there was no trial being held at the time of the destruction, the individual is not guilty of this particular offense. Moreover, the items that are destroyed or concealed need not be legal documents; anything that can be used as evidence in a trial is fair game.
This specific crime is classified as a misdemeanor offense and requires up to six months in a county jail in California and a possible fine. While this crime is specific to evidence during an ongoing trial or investigation, there are other related offenses. There are many other ways for an individual to provide false testimony or help the criminal/defendant get away with their actions through more illegal means.