With the Border Patrol pushing border crossers into more remote areas, and with a heat wave punishing parts of the Southwest, forensic researchers say the number of migrants dying around the border may actually be much higher, providing the Águilas with many grisly reports of such tragedies.

Sometimes the relatives of missing migrants accompany the Águilas on their grim task, as Mr. Luna did one recent Saturday, just an hour’s drive from prosperous San Diego. He joined more than 20 volunteers who piled into aging 4x4s and then fanned out on a stretch of barren federal land.

After trekking under the desert sun, using satellite images and intelligence gleaned from other migrants, they found the body of a man who seemed to have been trying to find shelter near a boulder in his last moments alive. Next to him, in bag of belongings, was a Mexican ID card with the name of Adrian Luna, who went missing two months ago while crossing the border.

“That was my brother,” Mr. Luna said after absorbing the shock of finding the remains, and thanking the Águilas for their help in finding the body. He and the volunteers placed a simple wooden cross where the remains were found. “Now, maybe it’s time to move on,” he said.