American missionaries Sam and Nancy Davis were crossing a drug-cartel battleground in northern Mexico when gunmen opened fire on their truck and mortally wounded Nancy.

Now the search for her killers, an investigation led by authorities in Tamaulipas, could be mired by the same violence as investigators in previous high-profile cases have turned up dead in this state bordering Texas, where two powerful cartels are warring for drug routes.

No law enforcement officials on either side of the border would speak publicly Friday about the security issue, or the fact that few crimes in Mexico ever get prosecuted.

"We expect the Mexican authorities to investigate this case," said U.S. Embassy spokesman Alex Featherstone. "We can't speculate on what might happen to the investigators."

The couple drove up to an illegal roadblock on a stretch of highway just south of the border city of Reynosa. When Sam Davis decided not to stop, gunmen opened fire and hit Nancy Davis, 59, in the head. Sam frantically drove his bleeding wife for help, crossing the wrong way on an international bridge into Texas, where she later died.

Tamaulipas will investigate the assault on the couple and the discharging of the firearm, said state Attorney General spokesman Ruben Dario Rios Lopez. But he said the state is not investigating a murder because Davis died in the U.S.

Her death Wednesday came four months after American David Hartley was gunned down on Falcon Reservoir bordering Texas and Tamaulipas as he and his wife tried fleeing on Jet Skis. Last summer's massacre of 72 migrants also took place in Tamaulipas.

A prosecutor and transit police officer investigating the migrant case both turned up dead. A state police commander looking into Hartley's disappearance was decapitated, his head delivered to a Mexican army post in a suitcase.

Hartley's body was never found and no one has been arrested.

"Falcon Reservoir was a different issue," Rios said. "Assistant Attorney General Hernan de la Garza is in charge of this investigation, and he's known as a very capable investigator."

State and local law enforcement in Texas, as well as the FBI, all pledged to do what they could to help the Mexican investigation.

The couple, who had been doing missionary work in Mexico for 30 years, was well aware of the dangers they faced and had cut back their trips recently as a result.

Intense violence has plagued all of northeastern Tamaulipas state, where the Gulf cartel and the Zetas have been battling for lucrative drug routes in the United States for more than a year.

The Mexican government in November sent more troops and federal police there in what it called a major operative to try to control drug violence.

Francille Davis said her son and daughter-in-law were in Mexico Wednesday to pay pastors in some of the village churches the family had established. She said the drug war had prevented Sam from reaching the churches earlier in the month.

Among their stops was the tiny hamlet of Villa Mainero, where the Davises had a church. Residents said the couple was well-known and well-loved for work they had done over decades.

They ran a children's shelter for some time and founded several churches in other small settlements. Nancy worked as a nurse and midwife, gave Bible classes and was known as a deeply religious person who used music to express her faith.

Adelaeda Martinez, now 42, grew up in the children's shelter, where her mother sent her so she could go to school. She said considered Nancy her spiritual mother.

"My mother died a month ago and they came from the U.S. to be with us in a difficult time," Martinez said. "They gave everything in their lives to be here with us. Not everyone wants to come here because we're in the mountains, the road is difficult and it's not safe."

Like most small towns in Mexico, the pueblo has a central plaza and town hall with a few simple convenience stores and stalls that sell tacos and ice cream. Few residents have telephones in their homes, said housewife Olga Cepeda, and many leave in search of work, often finding their way to cities like Monterrey or across the U.S. border.

In a sign of the effects of Mexico's drug war, Cepeda said gunmen in flashy SUVs are now a common sight on the roads around the once-peaceful town. The highway that runs past Villa Mainero to Monterrey and north to the border with Texas is a notorious drug-smuggling route.

"They loved Mexico," Cepeda said of the missionary couple. "I think, sadly, that caused her death."


Associated Press Writers Linda Stewart Ball and Paul J. Weber in Texas and Mark Walsh in Monterrey, Mexico, contributed to this report.