For years, Napolitano and other officials at the Department of Homeland Security have pointed to the declining number of border apprehensions as proof that the total number of illegal crossings is also declining. Now, it could mean the administration just isn’t catching most of the crossers.

“The results speak for themselves,” says one GOP Hill aide involved in border security issues. “We can’t really use apprehensions as an accurate measure when we’re not even seeing half the people.”

In light of the radar numbers, McCaul has asked Napolitano to provide data to back up her assertion that the border is more secure than ever. The answer could have a huge effect on the comprehensive immigration reform bills Congress will consider in coming weeks and months.

For example, there are reports that the Senate’s bipartisan Gang of Eight negotiators have added a border security provision to their proposal to give immediate legalization to the estimated 11 million illegal immigrants in the U.S. Before that legalization occurs, Homeland Security would have to submit a plan that would, within a decade, result in the apprehension of 90 percent of those who cross the border illegally. The department would also have to have 100 percent of the border under surveillance.