The Cato institute is libertarian not left wing. As for the source data, you'll have to follow the links within the article.So they do not have an actual link to the source data?
I just looked up the Cato Institute. It is a far left organization, so without direct access to the data they are extremely suspect.
Especially given that honest people would go through the normal refugee process.
It's a well known statistic that illegal aliens commit less crimes than the average citizen. Perhaps it's influenced by their desire to not get caught here; therefore they want to avoid getting into legal trouble. Though it's just conjecture.