A decade ago, just 10 percent of the beds in the nation's civil detention system were in private facilities with little federal oversight. Now, about half the beds are part of a sprawling, private system, largely controlled by just three companies: Corrections Corporation of America, The GEO Group, and Management and Training Corp.

And the growth is far from over, despite the sheer drop in illegal immigration in recent years.

CCA was on the verge of bankruptcy in 2000 due to lawsuits, management problems and dwindling contracts. Last year, the company reaped $162 million in net income. Federal contracts made up 43 percent of its total revenues, in part thanks to rising immigrant detention.

GEO, which cites the immigration agency as its largest client, saw its net income jump from $16.9 million to $78.6 million since 2000.

At the same time, the three businesses have spent at least $45 million combined on campaign donations and lobbyists at the state and federal level in the last decade, the AP found.