Guess what crime is the most prosecuted at the federal level in the United States? Rape? Murder? Assault? Robbery? Racketeering? Financial fraud?

Actually, no. It's illegal entry and re-entry . And I don't mean people trespassing into your home or office. That would be scary indeed. What I mean is people being prosecuted for crossing our northern or southern border without authorization.

Given the Attorney General's recent articulation of the federal government's prosecutorial priorities – namely, national security, violent crime, financial fraud, and protection of our society's most vulnerable – this doesn't make a whole lot of sense. And despite what you may think, there's actually no solid evidence that these prosecutions actually deter future illegal migration, the Department of Homeland Security's assumptions notwithstanding (see here and p.17 here ).

Over the course of five years, the Obama administration has presided over about 400,000 criminal prosecutions of immigrants whom the federal government could have simply sent through the regular civil immigration system. But instead, the Departments of Justice and Homeland Security are cycling them first through the criminal justice system, pouring billions of taxpayer dollars into their prosecution and incarceration that could otherwise be spent fighting real public safety concerns.

These unnecessary incarcerations also add to the strain on a prison system that's already nearly 40 percent over capacity. Meanwhile, the immigrants themselves are slapped with federal criminal records that could stand in the way of their return to the United States – regardless of family, community, or economic ties in the country.

In FY 2013 alone, U.S. Attorneys' offices filed criminal charges against more than 90,000 immigrants for illegal entry or illegal re-entry under the federal criminal code – at an annual cost, for incarceration alone, that's been estimated at $1 billion .

The vast majority of these individuals were convicted, and eventually deported and counted among U.S. Immigration and Custom Enforcement's (ICE) annual removal numbers. But first, U.S. taxpayers spent billions of dollars on their criminal prosecution and their incarceration, including significant funding for district courts, U.S. Attorneys offices, federal public defenders, Criminal Justice Act panel attorneys, court interpreters, and prison. (Even so, their court proceedings fail to meet basic standards of due process .)

Recently, ICE – the federal government's interior immigration enforcement agency – has faced both criticism and praise for its fiscal year 2013 removal numbers, released just days before Christmas. ICE's 368,644 removals last year marked the lowest annual deportation level under the Obama administration, even as President Obama's Department of Homeland Security closes in on a record 2 million deportations, likely in the coming weeks.

But the fact is that these numbers don't tell the whole story. ICE is not the only player in the deportation game. In a prime example of misplaced priorities, our federal criminal justice system is playing an overzealous role in it too.