[zeestephen]

The author is playing a bit fast and loose with the facts.

Most of the non-USA citizens in federal prisons have committed a felony that is specifically related to immigration.

Since immigration crimes are not prosecuted at the state or local level, that means that 100% of immigration criminals are imprisoned at the federal level.

[donna]

. . . it depends on what question you want to answer. If you want to know how many noncitizens are in prison for breaking a law that all citizens must obey — like not committing murder as in San Francisco — then you’d probably prefer the lower number from the Bureau of Justice Statistics.

If you want to know how many prisoners broke any kind of law, including sneaking across the border, then you’d find the Bureau of Prisons number more useful.

http://www.politifact.com/punditfact/statements/2015/jul/10/larry-oconnor/conservative-host-noncitizens-are-25-federal-inmat/