SAN DIEGO - The first Border Patrol agent ever to die from apparent heat-related causes while on duty was buried Friday with a 21-gun salute.

Eric Cabral, a 31-year-old fitness buff, died July 26 after collapsing on solo foot patrol in a failed pursuit of suspected illegal immigrants in the rugged mountains east of San Diego.

Hundreds of migrants perish every year from heat and cold trying to sneak across the 1,952-mile U.S.-Mexico border, but Cabral is the first Border Patrol agent believed to have been killed by extreme weather.

"Your enemy out there is not only the bad guys, it's the weather," said Don McDermott, the supervisor of Cabral's Border Patrol station. "It's like working in an oven."

Cabral's physical stamina drew the attention of instructors at the Border Patrol Academy in Artesia, N.M., where he graduated in February 2006.

"The physical trainers would yell at Eric because they thought he mocked their workouts," Julius Alatorre, a fellow Border Patrol agent and trainee, told about 500 mourners at St. Michael Catholic Church.

The temperature hit a high of only 95 degrees about three hours before Cabral collapsed but the air was muggy with scattered rain, said Philip Gonsalves, a National Weather Service meteorologist. Summer winds from Mexico typically bring tropical moisture to southwestern states.

In heavy humidity, sweat doesn't evaporate, trapping heat and causing body temperature to rise, Gonsalves said.

The San Diego County coroner's office has not determined a cause of death but Border Patrol officials have said Cabral exhibited signs of heat exhaustion or heatstroke.

Seismic sensors alerted Cabral and a colleague around 9 a.m. to a group of suspected illegal immigrants near Jacumba, a hamlet 60 miles east of San Diego that is surrounded by boulders, cactus, manzanita trees and sage brush.

Working from separate vehicles, the agents walked alone carrying heavy gear in areas without roads and kept in touch by radio, said Richard Smith, a Border Patrol spokesman. The pair abandoned the pursuit after seven hours. They met briefly, and headed separately back toward their cars.

When Cabral never showed up at a roadside meeting point, another agent who was assigned to pick him up launched a search, Smith said. Cabral was found unresponsive, with a cut on his forehead and water in his canteen. He was flown to a hospital, where he died less than three hours after his partner last saw him in apparently good shape.

Agents often walk the border solo, looking for broken twigs, footprints, overturned pebbles and other clues. They are aided by ground sensors and communicate with colleagues by radio.

At least 4,500 migrants have been found dead along the border since a U.S. government began in the mid-1990s that pushed traffic to remote deserts and mountains, said Claudia Smith of the California Rural Legal Assistance Foundation, a migrant advocacy group. Smith said that tally came from records she obtained from the Mexican government, but that the actual toll is even higher because some bodies are never found.

T.J. Bonner, head of a union that represents Border Patrol agents, said he was surprised that no agents had died sooner, considering how many migrants have perished.

Bonner, president of the National Border Patrol Council, suffered heat exhaustion twice while on duty, including once while working the mountains east of San Diego in the 1980s. He said he was bedridden for three days and felt "weak as a kitten."

Since 1919, 103 Border Patrol agents have died on duty, according to The Officer Down Memorial Page Inc., which tracks law-enforcement deaths. Gunfire was the leading cause with 30 deaths, followed by automobile accidents and aircraft accidents.

Cabral, a San Diego native, earned a master's degree in physical education from Azusa Pacific University in 2003 and taught health and fitness at Southwestern College in Chula Vista, a San Diego suburb, before joining the Border Patrol.

On the Net:

The Officer Down Memorial Page's tally of Border Patrol deaths:

http://www.odmp.org/agency.php?agencyid4830