Her husband, Samuel Davis, piloted his bullet-ridden truck across the two-mile international bridge here, driving pell-mell against traffic on the wrong side of the bridge to evade the pursuers and reach an American hospital. He arrived on the United States side too late to save Ms. Davis, 59.

State Department officials say that 79 American citizens were murdered in Mexico in 2009, and that at least 60 were killed last year from January to November, though an official annual figure has yet to be compiled. The numbers have been rising since 2007, when 38 American citizens were murdered in Mexico, State Department records show.

In late September, an American man was shot to death while he and his wife were riding water scooters on the Mexican side of Falcon Lake. A month later, a student at the University of Texas, Brownsville, was taken off a passenger bus and killed by gunmen. Then in November, four men from San Marcos, Tex., along with a 14-year-old visitor from Chicago, disappeared in Nuevo Laredo and are presumed to have been abducted, the F.B.I. said.

The heaviest toll is in El Paso, where many residents cross the border regularly to conduct business or visit family.

In early November, for instance, four American citizens were killed in separate crimes over one weekend, including a 15-year-old boy. All of the victims were ambushed and shot to death while visiting Ciudad Juárez, which has become one of the most murderous cities in the world because of a battle between the Sinaloa cartel and the remnants of the Vicente Carrillo Fuentes gang.

“We know that many of our winter Texans enjoy traveling to Mexico, but they should understand that we cannot guarantee their safety after they cross the border,” Steven C. McCraw, director of the Texas Department of Public Safety, said in a warning issued after Ms. Davis’s death.

Relatives described Ms. Davis as an ebullient and devout woman who loved working with people in rural Mexico. She was a registered nurse and had worked as a midwife in Mexico, in addition to teaching Bible classes. She also composed religious songs on the piano and sewed her own prairie-style dresses.

For decades, she and her husband had run a charity — the nondenominational Gospel Proclaimers Missionary Association in Weslaco, Tex. — that raised money to build churches, hold revivals and distribute Bibles in poor Mexican villages, mostly in the states bordering Texas.

About a year ago, Ms. Davis and her husband moved their base of operations from a small Mexican town in Nuevo León State to their house in Monte Alto, Tex., where they had raised their two grown sons. They had also curtailed their trips to Mexico in recent months after having some close scrapes involving highway robbers, said Melody Reynolds, a niece of Mr. Davis’s.

But last week the couple had received a message from the pastor of one of the churches they had established during their 30 years of missionary work, Ms. Reynolds said.

The pastor said the church was in financial trouble and needed cash. The couple generally drove an older model car while in Mexico to avoid attracting carjackers, but that vehicle was in the shop, so they took their 2008 Chevrolet pickup truck. The police in Pharr say they think that choice made them a target.

The trip took three days, and the couple were on their way home when a group of men brandishing guns began tailing them, Ms. Reynolds said.

They were just outside San Fernando, 87 miles south of the border. It is a region that has been plagued over the last year by battles between the Gulf cartel and the Zetas.

Mr. Davis decided to run for it, but the truck behind him caught up. Several miles later, two other trucks tried to block the road, but Mr. Davis managed to get past them, Ms. Reynolds said.

The Pharr police chief, Ruben Villescos, said the motive for the attack remained a mystery. He said the men in the three trucks followed Mr. Davis for miles and boxed in his pickup to force him off the road. Several shots were fired, and Ms. Davis was hit in the back of the head.

One slug went through the passenger side window and through the windshield near Mr. Davis, Ms. Reynolds said.

“He says to this day he doesn’t know why he’s alive,” she said. “He got shot at. Apparently it wasn’t his time to go.”