TRAC shared its findings with both ICE and CBP and asked for comment for these declining numbers. A CBP spokesman explained this decline as a direct result of the fall in border apprehensions:

"We believe that the investment the American people have made in personnel, infrastructure and technology on the borders of the United States are continuing to pay dividends in the form of lower apprehension numbers."

ICE-referred criminal prosecutions have been falling at a much faster pace over the past 12 months than those from CBP — down 15.2 percent for ICE as compared to a decline of 7.9 percent for CBP (see Table 1). We therefore asked ICE more than a week ago "if there is any particular reason(s) accounting for this recent decline, since other aspects of enforcement activities — such as actual removals and returns — have been going up." ICE did not respond to the queries.

At the time President Obama took office in January 2009, the overall number of criminal prosecutions resulting from CBP and ICE referrals to federal prosecutors had reached a peak of almost 100,000 per year. Since then, prosecutions have fallen by 16.8 percent. (Note that while the January 2009 total was 97,961, a 12-month peak of 98,727 CBP-ICE referred criminal prosecutions was reached in March 2009. Of course these prosecutions still largely reflected investigations completed during the last year of the Bush presidency.)

For the 12-month period ending August 2012, nearly four out of five (78.2 percent) referrals came from CBP; most of these originated as Border Patrol apprehensions. The remaining one-fifth (21.8 percent) were due to ICE-led investigations.

Attention Given to the Southwest versus Northern Border

Table 2 shows that virtually all (98.4 percent) of CBP's enforcement efforts are focused along the southwest border, which is composed of five federal districts: the Southern District of California, Arizona, New Mexico, and the Western and Southern Districts of Texas. CBP's focus has changed little during the past four years; when Obama assumed office in January 2009, 98.2 percent of CBP criminal prosecutions originated in these southwest border districts.

While much longer than the border with Mexico, the non-Alaskan northern border between the U.S. and Canada was the focus of less than 1 hundredth (0.9 percent) of CBP's criminal enforcement activities. This despite the fact that it encompasses prosecutions across 14 federal districts that border Canada or the Great Lakes between Canada and the U.S. Less surprisingly, the remaining 71 federal judicial districts also saw little activity, accounting for only 0.8 percent of the total.

ICE's enforcement mandate covers the entire nation, including the judicial districts in the interior of the United States. But during the last 12 months, more than half (52.3 percent) of the criminal prosecutions resulting from its investigations occurred in the five southwest border districts (see Table 3). In contrast, when President Obama first took office such prosecutions made up just 41.4 percent of the total.

Looked at from another perspective, this means that ICE enforcement in the interior districts had dropped from being the primary focus down to only 41.1 percent during the last 12 months.

The fourteen federal districts along the northern border received much less attention from ICE. Even this minor attention has slipped since President Obama assumed office — falling from 7.6 percent of the total to 6.6 percent (see Table 3).

District-by-District Criminal Prosecution Numbers

During the Obama administration thus far (January 2009 through August 2012), there have been a total of 329,889 criminal prosecutions resulting from CBP or ICE referrals. The number of prosecutions for each of the federal judicial districts is provided in Table 4.

With 95,679 criminal prosecutions, the Southern District of Texas accounted for almost three out of ten. Arizona was second with 91,122. Farther behind in third place was the Western District of Texas with 61,754. Rounding out the southern border districts were the Southern District of California and New Mexico, with 21,542 and 19,856 criminal prosecutions respectively.

Along the northern border, the Western and Northern Districts of New York were in first and second place, with just over 1,000 prosecutions each. In third and fourth place were the Western and Eastern Districts of Washington, with 739 and 658 criminal prosecutions, respectively.

Somewhat surprisingly there were other districts, neither on the border with Canada nor considered major international travel centers, that had a high number of enforcement actions. Among the standouts in this regard were Utah with 1,322 prosecutions and Oregon with 1,037. See Table 4 for the prosecution counts for all of the districts in the country.