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

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

«The Grupo Escorpiones Gulf Cartel strongly condemns the events of Friday, March 3, in which an innocent working mother unfortunately died and four US citizens were kidnapped, two of whom died,» reads the translation of the letter. «For this reason, we have decided to hand over those directly involved and responsible for the events that at all times acted under their own determination and indiscipline and against the rules in which the CDG has always operated.»

The four Americans were shot and kidnapped after crossing the border into northern Mexico to undergo a medical procedure. The governor of the state of Tamaulipas, Américo Villarreal, said in a press conference that a woman, identified by the family as LaTavia Washington McGee, and a man, Eric Williams, survived the kidnapping.

Both were taken to a clinic for medical treatment and returned to the United States via an international bridge between Matamoros, Mexico and Brownsville, Texas.

This is a developing story. Please check back for updates.