It's very common for illegal aliens to commit a crime such as murder (95% of outstanding homicide warrants target an illegal alien) and then flee to Mexico. California prosecutors can ask for extradition, but Mexico won't extradite anyone who may face the death penalty. However, once the Mexican authorities are aware of a crime that was committed in California, they may decide to prosecute it themselves, get a conviction, and then impose a rather light sentence (by American standards). For example:
Another example involves Mario Abendano Chaidez who shot and killed 17-year-old Francisco Barajas Lopez in Los Angeles on November 8, 1989 . The murder occurred after Chaidez lured Lopez out of his house to ask him about Lopez' telephone calls to Chaidez's daughter. After a brief conversation, Lopez walked away from Chaidez and was shot in the back of the head.
Chaidez then fled to Mexico where he was acquitted of murder then tried and convicted of manslaughter. Chaidez ultimately served two years in custody before being sentenced to an additional eight years in which he was required to serve time on weekends only. In California , Chaidez would have faced a murder charge and a possible sentence of 35 years to life.
The problem is that under current California law, if the criminal returns to the state we can't prosecute him for the crime anymore -- he's already been convicted in Mexico. (This isn't because of the double jeopardy prohibition in the Constitution.)
The end result is that prosecutors are wary of seeking extradition because they're often afraid that the request will be denied and that they'll then lose any opportunity to get a conviction later because Mexico will try the case itself (and give a lenient sentence).
Somewhat confusing, I know. California Assembly Bill 1432 seeks to amend the controlling statutes to allow authorities to prosecute suspected felons even if they've already been tried in another country. If it passes, it would likely lead to more extradition requests because it would remove one of the major disincentives. It doesn't mean more extraditions would be granted, but at least if criminals return to America we'd be able to go after them.
On an unknown date, Chaidez re-entered California before being arrested on a warrant for the 1989 murder in October 2002. Due to current California law, Chaidez's warrant was quashed and his case was dismissed. If Chaidez is arrested again and convicted of a felony, the three-strikes law and the five-year sentence enhancement for a prior serious felony conviction would not apply under the present law.