It was after dusk in mid-December, and the dark waters of the Colorado River were below 50 degrees. Fan Tuan Gao, 19, an illegal immigrant from China, was in the river trying to sneak into the United States with four Chinese companions and two Mexican smugglers.

Witnesses recalled that Fan was drowning in the fast-rushing waters and screaming for help when U.S. Border Patrol Agent James Epling waded into the deep river and carried her to safety.

Epling then turned to go catch the others, who were swimming and paddling in inner tubes to try to get away. Agents heard him make a last call on his radio: "I'll be off the radio just a minute," he said, before shutting it off so it wouldn't short out in the water.

Then he ran off in the shallows along the thick reeds and brush, into the darkness.

Divers found Epling's body three days later, drowned at the bottom of the river less than 200 yards from where he had disappeared, still wearing his Border Patrol baseball cap, still clutching his flashlight.

Except for Epling's death -- the seventh in 40 years for the Border Patrol's Yuma station -- the story of how a 24-year-old former college track star from Virginia and a teenager from rural China ended up together in a frigid river near Mexico is numbingly routine.

Border Patrol officials said their officers catch an average of 2,500 people a day -- more than 100 an hour -- who, despite sharply increased security since the Sept. 11, 2001, terror attacks, continue to try to sneak across the U.S.-Mexico border. The Border Patrol, a division of the Department of Homeland Security, has 9,800 agents stationed along the border, up from 3,600 a decade ago. That includes 1,300 agents hired since 2000, reflecting a new emphasis on tracking border crossings.

Agents in darkened control rooms monitor hundreds of infrared video cameras that keep an electronic eye on the border as if it were a 2,000-mile-long convenience store. Hundreds of motion sensors are hidden in the scrub brush. Each time an unsuspecting immigrant passes one, an alarm goes off in a control room -- programmers made it sound like a door bell, a bit of borderlands humor: It's the sound of another guest arriving.

'He Loved the Chase'

The last day of Epling's life started out with pre-Christmas errands, said Epling's widow, Monica. While she shopped, her husband, who worked the 3 to 11 p.m. shift, played with their three young boys. Monica Epling was nearly eight months pregnant at the time; she gave birth to James Paul Epling II on Jan. 28.

Christmas was especially important in their house, because two of the boys were born on Christmas day: Sean was nearly 10 and Shaine nearly 4; Seth had just turned 2. When Monica returned home, she said they paid bills and went over gifts they still needed to buy. The family had lunch together, then Epling left for work.

Monica said her husband joined the Border Patrol in January 2003 partly on the urging of her father, a retired agent. She said law enforcement was a good fit for Epling, who came from a military family. He was a member of the U.S. Army Reserves. His father is a retired U.S. Navy officer and his two younger brothers are both in the military -- including one in the Army's elite 82nd Airborne Division who just returned from Iraq.

Epling was 6 feet 2 and a competitive runner and swimmer, and Monica said it was the physical aspect of his job that he loved the most -- the actual chasing after illegal immigrants. He excelled in track in high school in Williamsburg, Va., and at Christopher Newport University in Newport News, Va., and he finished first in his class in physical training during Border Patrol boot camp.

"He loved the chase, them thinking that he wouldn't catch them," she said. "He loved the challenge."

The flow of illegal immigrants across the desert badlands here has become an issue in the U.S presidential race. President Bush has proposed giving temporary visas to millions who crossed the border illegally and are now living in the United States. His plan has caused frustration among many Border Patrol agents, whose union called the plan "a slap in the face."

Many agents said they now wonder why they risk their lives to capture illegal immigrants who could soon be rewarded with legal status. Their job has become increasingly dangerous: Of the 93 agents who have been killed in the service's 85-year history, 18 were killed in the 1990s -- more than twice the average in the previous five decades.

But Monica said her husband never questioned the value of his work. She said he believed in his job capturing illegal immigrants because "there are legal ways to enter the United States."

China to N.Y., Via Mexico

Fan told investigators that she and her companions were from Fujian province, on China's southeastern coast across the strait from Taiwan. She said they flew from China to Singapore to Rome to Mexico City -- a trip of more than 13,000 miles. Then, she said, a Mexican smuggler met them at the airport and brought them nearly 2,000 miles more to the border. She said they went first to a house in the desert, where they stayed for four days before trying to cross the border.

The vast majority of the border-crossers are from Mexico, but last year officials said nearly 50,000 came from 150 other countries; nearly 1,000 Chinese citizens were caught last year trying to cross the border, according to Mexican and U.S. officials.

Tens of thousands of people every month continue to find the economic allure of the United States outweighs the enormous costs and dangers of trying to cross. Analysts said Chinese immigrants such as Fan pay as much as $70,000 for the trip -- about 70 times the average annual income in her native province. In the past five years, at least 2,000 people have died crossing the border, but they still come.

Through her attorney, Fan declined to be interviewed. In her statements to investigators, Fan, 5-2 and 111 pounds, said she and her boyfriend, Jian Qiu Chi, 21, were both from Fuzhou and their companion, Li Da, 18, was from Changle, a nearby town.

Many people from Fujian now have relatives established in the United States who offer them a destination and help getting started, said Peter Kwong, a leading specialist in Chinese immigration at Hunter College in New York.

Fan told investigators she had hoped to go to New York -- the initial destination of most Chinese immigrants, Kwong said. Fan and her companions most likely would have spent the next few years working 80 or 90 hours a week in the garment industry, restaurants or construction to pay off their debt to the smugglers, he said.

But that plan ended abruptly when a Border Patrol agent spotted them in the river and radioed Epling for help.

Fan and the two others captured by Epling are now in custody near San Diego, held as witnesses against accused smuggler Jose Antonio Vazquez Villasenor, 22, an auto mechanic from the border town of Los Algodones, Mexico, a few miles across the border from Yuma. He was caught less than two hours after Epling disappeared when he tripped a motion sensor on the riverbank. According to court records, he said he earns $150 a month fixing cars, and he's been smoking marijuana or using crystal methamphetamine daily for 10 years. If convicted of the most serious charges against him, human smuggling resulting in death, he could face the death penalty.

A Dash, Then Silence

Epling arrived at the Colorado River in his Border Patrol truck in the gathering darkness. About a quarter-mile north of Mexico, on the border between Arizona and California, the area is remote, with no fences. Immigrants often paddle up the river and come ashore here, then dash across the vast lettuce fields.

The riverbank is covered with thick reeds and vines that are up to 15 feet high. Agents who investigated Epling's death said Fan and her group apparently used that brush as cover, making their way against the current from Mexico on inner tubes, carrying their clothes in black plastic garbage bags. On the bank near where they landed, little niches in the brush are littered with deflated tubes, garbage bags and discarded underwear.

Agent John Bullington and two other agents arrived at the river shortly after Epling. He said the people in the river were hiding where the brush was especially thick, so the agents had to hack and push through about 100 yards of it to get to them. He said it was an exhausting slog that took nearly an hour.

As they finally neared the river, Bullington said Epling spotted the group.

"I've got the bodies," he heard Epling shout, using border slang for illegal immigrants.

Bullington said Epling grabbed two Chinese men on the bank and passed them to Agent Jon Shelton.

"Then all of a sudden, I heard a woman crying," Bullington recalled.

Trying to flee the Border Patrol agents, Fan and the others rushed back into the water. While the others swam off, Fan apparently slid down the steep river bottom and was suddenly in trouble. Bullington said Epling waded deep into the river, fighting against the current to pull her ashore.

Shelton watched as Epling ran off down the bank, splashing through the shallows. "He was moving so fast, it was almost like he was running on the water," Bullington said.

No one knows exactly what happened. Postmortem exams showed that Epling drowned; there were no signs of foul play. The sharp river bottom drop-off, the cold water, the strong current, the darkness and the gnarly brush that made it difficult to climb out of the water are all suspected factors. Epling was also wearing at least 30 pounds of gear, including a bulletproof vest, military-style boots, a gun belt, gun, ammunition, radio and flashlight.

"We never heard him call for help," Bullington said. "We never heard a splash in the water, we never heard anything."

A Border Patrol helicopter searches the Colorado River, where Epling rescued a Chinese woman from drowning and then went back to chase her companions. James Epling dashed into the Colorado River in pursuit of illegal immigrants. At right, a Border Patrol dive team examines the area where he was last seen.