(CNN) -- An American missionary was fatally shot in Mexico on Wednesday, police said.
The preliminary investigation indicated that Nancy Davis, 59, and her husband were traveling on a Mexican highway near the city of San Fernando, Mexico, when they were confronted by gunmen in a black pickup, the Pharr Police Department in Texas said in a statement. San Fernando is south of the border city of Reynosa in Tamaulipas state.
"The gunmen were attempting to stop them and the victims accelerated in efforts of getting away from them," the police statement said. "At a certain point the gunmen discharged a weapon at the victim's vehicle and a bullet struck the victim Nancy Shuman Davis on the head."
Davis' husband, identified as Sam Davis by family friends, drove their truck "at high rate of speed" to the Pharr International Bridge, which crosses the Rio Grande. Nancy Davis was taken to a hospital in nearby McAllen, where she was pronounced dead about 90 minutes later.
The Texas Department of Public Safety, Immigration and Customs Enforcement, the FBI and U.S. Customs and Border Protection are also assisting, police said.
The Mexican Interior Ministry said it, too, was investigating "for the purpose of clarifying the facts and act accordingly to punish the person or persons responsible."
Merton Rundell, a professor in the missions department at Indiana's Union Bible College & Academy and a family friend, told CNN the couple had lived in Mexico since the 1970s and worked with the Gospel Proclaimers mission in Mexico.
Maryanne Wheeler, another friend who worked with the Davises in the 1990s, said Nancy Davis' death was a great loss.
"They breathed Mexico," Wheeler said. "That's their love.
"For 40 years she has gone around Mexico, trying to be there as a nurse, a friend, as a spiritual adviser and has loved them. They lost the best," she said. "They had a petite lady who had the tenacity of a bulldog and was there for you."
Wheeler said the couple had been shot at before and knew the dangers of the border area.
CNN's Sara Pratley, Anna Gonzalez and Nick Valencia and CNN Mexico's Krupskaia Alis contributed to this report