NEW : Mexican police say they are dead . Nancy Davis was killed by gunmen in a black car , police say . They lived in Mexico since 1970s ; they worked with Gospel Proclaimers mission . ` ` They love Jesus . That 's their love , '' friend said of the couple .
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 .