On the weekend before Christmas, a group of volunteers set out to search for the remains of dead migrants on the Barry Goldwater Air Force Range, a remote desert area in southwestern Arizona where military jets from bases in Arizona conduct live bombing exercises.

The volunteers from the non-profit group Aguilas del Desierto recovered the skeletal remains of eight migrants in two days.

The San Diego-based group posted pictures of some of the remains it recovered near Ajo on its Facebook page, including a skull, a jaw and a set of leg bones, bleached white by the sun.

The photos were reminders that high numbers of migrants continue to die annually in southern Arizona after crossing the border illegally, as increased border security over the years has pushed some migrants to take more treacherous treks to avoid being caught by the Border Patrol.

In 2018, the remains of 127 dead migrants were recovered in southern Arizona, the Pima County Medical Examiner's Office said.

The office conducts autopsies on the remains of migrants found in Pima, Cochise and Santa Cruz counties, where the majority of migrants die.

The 127 remains recovered in 2018 were slightly below the 128 remains recovered the previous year, Medical Examiner's Office data show.

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Over the past two years, the number of migrants recovered in southern Arizona has dipped to some of the lowest levels in recent years. In 2010, the number of migrant deaths recorded by the Pima County Medical Examiner's Office peaked at 222.

Although the number of remains recovered dipped in 2018, "we are not down to where we were," said Gregory Hess, Pima County's chief medical examiner.

Deaths in desert went up after 2000

Before 2000, the remains of fewer than five migrants were found each year, the Arizona OpenGIS Initiative for Deceased Migrants shows.

In 2000, just one was found. In 2001, the number soared to 79 and then to 151 the year after that. The number of annual migrant deaths since then has remained well above 100, according to Humane Borders, a Tucson-based group that compiles data and logs it on the OpenGIS website.

From 2001 through 2018, the remains of at least 3,011 dead migrants have been recovered in southern Arizona, according to the Humane Borders website.

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Migrant deaths have remained high despite a sharp decrease in the overall number of migrants apprehended by the Border Patrol, which in fiscal year 2018 totaled just less than 400,000, down from 1.6 million in 2000, when Border Patrol apprehensions peaked.

Of the 127 remains turned over to the Pima County Medical Examiner's Office in 2018, 91 remain unidentified, and 36 have been identified, according to The Arizona Republic's analysis of data provided by the office.

Ninety-seven of the 127 remains have been identified as male, and 10 as female, the analysis showed. The gender of the remaining 20 had not been determined, the data show.

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Of the 127, 86 remains found in 2018 were skeletal or partially skeletal, with the cause of death unknown, the data show. Another 25 migrants died of probable hyperthermia, the data indicated. Hyperthermia is when the human body becomes too hot.

Group looks for remains once a month

Aguilas Del Desierto conducts searches about once a month on federal lands in southwestern Arizona, including the Organ Pipe Cactus National Monument and the Cabeza Prieta National Wildlife Refuge, said Vicente Ruben Rodriguez, a volunteer spokesman for the group.

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The searches are usually conducted after the group receives information from family members about migrants who have gone missing after trying to enter the United States illegally by crossing through the desert.

"Sometimes we find nothing," he said.

In December, the group received the military's permission to search the bombing range, he said. About 30 volunteers participated in the search, aided by a dog trained to sniff out cadavers, he said.

The group also has found high numbers of remains on several other recent searches on the bombing range.

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The high numbers of remains found on the bombing range during recent searches suggest the area has been a well-traveled corridor for migrants despite the dangers, and has not been effectively searched in the past, he said.

But it's difficult to tell how popular the range remains because several of the skeletons discovered appeared to be several years old, he said.