The Gulf cartel apologized Thursday after two American citizens were killed during an armed abduction last week in Mexico. A senior law enforcement official told NBC News that U.S. authorities believe the letter is legitimate. Four Americans were shot at and kidnapped after driving into Matamoros, Mexico, just south of Brownsville, Texas, for a medical procedure. Video showed a gunman dragging people into a white pickup, a law enforcement source with knowledge of the matter previously said. A law enforcement official with knowledge of the matter said a woman in the group had been seeking a cosmetic medical procedure.

---