There are other factors that contribute to this issue for sure, but lead was 100% one of those factors.

We 100% know that lead is a strong neurotoxin that especially fucks the minds of young adults and children. That is why leaded petrol which is more efficient than unleaded one, has been banned worldwide (not banned in only 1 country in the entire world). 

We also know that kids with fucked up brains (low IQ) are more likely to commit a crime.

There you have a causal link. How is it far from being proven?