A special team of immigration advocates, working in collaboration with forensic experts, will be in Manhattan later this month to get DNA samples from New York City area immigrant families whose relatives are missing and may have died crossing the deserts of the American southwest.

While the death of 10 migrants in a sweltering tractor trailer in San Antonio a few weeks ago is the latest example of perils associated with human smuggling, thousands of immigrants have been turning up dead for years as they try to cross the U.S. border areas of Arizona and Texas on foot, experts say.

“I would characterize the scope of the loss of human life on the border as catastrophic,” said Robin Reineke, co-founder of the nonprofit Colibri Center for Human Rights in Tucson, Arizona, the group organizing DNA collection effort in the city. “The true number of dead and missing are unknown, but are easily above 10,000.”

Another expert said forensic teams excavating cemeteries in Texas sometimes find four bodies in a grave. “There are huge numbers of deaths which nobody is paying attention to,” said Lori Baker, an official of Baylor University who in 2003 started the Reuniting Families Project to help identify remains of immigrants.

Reineke said that her staff has information that more than 200 New York City families are trying to find missing relatives with whom they lost contact during smuggling attempts. Colibri staff will be in Manhattan from Friday to Aug. 15. Because of concerns about the undocumented status of some of the families, Colibri is not identifying the sampling location.

Colibri’s push to get DNA reference samples from New York families is part of a wider effort to overcome the reluctance of immigrants to cooperate with law enforcement agencies that have the ability to check DNA profiles with those already on file from retrieved bodies. In the past, the University of North Texas would generate DNA profiles to compare with the law enforcement database known as CODIS, which Reineke said requires genetic profiles be submitted from police agencies.

But early this year, the National Institutes of Justice diverted funding from DNA testing of missing migrants to the screening of a nationwide backlog of rape kits.

Colibri gets around the CODIS restriction by sending its collected samples to the private Bode Cellmark Forensics in Virginia. Bode Cellmark has the largest private collection of DNA profiles for unidentified bodies found in Arizona, Reineke said. She explained that Hess’s office, in collaboration with the Mexican Consulate, has sent samples for all unidentified remains to Bode Cellmark since about 2002, creating a large database for comparison.

In New York City, the chief medical examiner in 2014 got a DNA hit and identified Manuel Merchan, a 33-year-old man from Ecuador whose remains were found in Brook County, Texas, in 2015. Merchan’s family lives in Westchester County.

In Pima County, Arizona, the remains of 2,600 migrants, known as “undocumented border crossers,” have been collected since 2002, said Dr. Greg Hess, the county’s medical examiner. Hess said he handled 154 bodies last year, a relatively small part of his annual caseload of 3,000.

“It is about the same type of situation like having a plane crash a year,” Hess said of the dead migrant workload.

Many remains are nothing more than skeletons and sometimes just bone fragments. Some carry identification documents but they may be forgeries. Dental records are often of no use since some countries have poor record keeping practices. Tattoos, styles of clothing and fingerprints sometimes provide leads.

Bones may be scattered — even ants may take fragments into their tunnels, said Jason DeLeon, an anthropologist at the University of Michigan who studies migrant deaths.

During one hike through the Arizona desert, De Leon and his colleagues came across the decomposing body of an Ecuadorean woman. The remains were identified by fingerprints as that of Carmita Maricela Zhagui, 28, who was trying to join her family in Queens, De Leon said.

People interested in giving DNA samples to the Colibri Center during its New York City area initiative 15 can contact the organization by sending an email to info@colibricenter.org.