A member of the Attorney General's Office of the State of Tamaulipas guards outside its building ahead of the transfer of the bodies of two of four Americans kidnapped by gunmen to the U.S. border, in Matamoros, Mexico, March 8, 2023.

The Gulf cartel apologized Thursday after two American citizens were killed during an armed abduction last week in Mexico.

The cartel condemned the violence in a letter obtained by The Associated Press and said they had turned over members who were involved. A senior law enforcement official told NBC News that U.S. authorities believe the letter is legitimate.