( 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 .
NEW : Mexican authorities say they are investigating . Nancy Davis was shot by gunmen in a black pickup , police say . Davises lived in Mexico since 1970s ; they worked with Gospel Proclaimers mission . `` They breathed Mexico . That 's their love , '' friend said of the couple .