Not only has the economic downturn in the United States eliminated many of the jobs that used to lure immigrants, human rights groups say, but also the federal government has stepped up efforts to stop the underground railroad of migrants, building mammoth fences in several border towns and flooding the region with hundreds of new Border Patrol agents equipped with high-tech surveillance tools.

These tougher enforcement measures have pushed smugglers and illegal immigrants to take their chances on isolated trails through the deserts and mountains of southern Arizona, where they must sometimes walk for three or four days before reaching a road.

“As we gain more control, the smugglers are taking people out to even more remote areas,” said Omar Candelaria, the special operations supervisor for the Border Patrol’s Tucson Sector. “They have further to walk and they are less prepared for the journey, and they don’t make it.”

Mr. Candelaria said the surge in discoveries of bodies this year might also owe something to increased patrols. He noted that some of the remains found this year belong to people who died in previous years. But Dr. Parks said that could not account for the entire increase this year. Indeed, the majority of bodies brought in during July, Dr. Parks said, were dead less than a week.

Human rights groups say it is the government’s sustained crackdown on human smuggling that has led to more deaths.

“The more that you militarize the border, the more you push the migrant flows into more isolated and desolate areas, and people hurt or injured are just left behind,” said Kat Rodriguez, a spokeswoman for the Coalición de Derechos Humanos in Tucson.