Kathryn Ferguson edged through a thicket of tangled mesquite trees and spotted a man's legs sticking out from under some low-hanging branches.

"Oh my God!" she uttered, freezing in her tracks because at first she thought the man was dead. He was merely sleeping as he and two other illegal immigrants took a break from their journey across this stretch of desert.

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Ferguson, a volunteer for a humanitarian group that tries to prevent deaths among illegal immigrants, expects to have help soon. Her organization and others say the national debate on immigration has renewed interest in their efforts, bringing many offers of assistance and donations.

"It's like this: If a major airliner crashed in the Arizona desert every year, people would be very upset about it," Ferguson said. "But it's only recently that people have taken note that 300 people die every year, like an airliner."

The escalating interest coincides with the onset of summer — the deadliest time for illegal immigrants crossing into the Southwest.

Border deaths have increased in recent years as migrants brave increasingly hostile terrain to avoid intensified enforcement in other areas.

A record 473 illegal immigrants died while trying to cross the U.S.-Mexico border during the fiscal year that ended Sept. 30, according to the Border Patrol. Many of the victims succumbed to the desert heat.

Groups such as the Tucson-based Samaritans send out patrols carrying food and water. If they find someone needing medical assistance, volunteers call the Border Patrol.

During a recent patrol, Ferguson pushed through brush about 25 miles north of Tucson, calling out in Spanish, "Good morning! We come to help ... We have food and water, medical attention."

She and a colleague offered to help the three men they found napping, but the migrants refused. For the rest of the day, they found discarded water bottles and backpacks with the contents strewn on the desert floor — toothpaste, deodorant, shoes, socks — but no other migrants.

The Rev. Robin Hoover of Humane Borders, a group that maintains 81 desert water stations, said church youth groups and other volunteers from Massachusetts, New Mexico, Minnesota, Virginia and Kansas are among at least a couple hundred people who will come to Arizona this summer to help.

Interest has risen every year since the group began in March 2001 with two water stations, Hoover said.

"It snowballs," he said. "More people get on our Web site, more send hate mail, more people send checks."

In San Diego, Enrique Morones said he has seen a similar spike in volunteers for Border Angels, a group that distributes water for migrants at about 300 sites in the Imperial Valley, east of San Diego, and in Baja California.

Morones said the group has more than 1,000 volunteers, with hundreds added in just the last few months. He anticipates getting another thousand over the next year. People often volunteer after hearing about migrant deaths along the borders.

"They go 'Wow! That's horrific!' Where before, people were dying and no one was hearing about it," Morones said.

Maryada Vallet, an organizer with No More Deaths, a coalition of religious groups and human rights advocates that organized in Arizona in 2004, said her group hopes to train 500 more new volunteers this summer.

But along with the rise in volunteerism there have also been obstacles.

Two members of No More Deaths face trial in October on charges of transporting an illegal alien. The pair say they were arrested while taking migrants for medical treatment.

No More Deaths is revising its protocols to ensure volunteers do not risk breaking the law. Aid workers must now call 911 and the Border Patrol rather than taking the immigrants to get medical treatment.

Legislation passed by the House in December also would make it a felony to assist someone trying to enter or stay in the country illegally.

Susan Wysoki, a spokeswoman for the Federation for American Immigration Reform, which advocates tighter border security, acknowledged the groups' humanitarian role, saying "there's no overstepping boundaries when saving lives."

But, she added, "the bottom line is that we don't want to encourage illegal behavior by encouraging these people to cross."