The Gulf cartel apologized Thursday after two US citizens were killed in 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 Cartel del Golfo Grupo Escorpiones 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,» the translation of the letter reads. «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.»

Four Americans were shot and kidnapped after they entered Matamoros, Mexico, just south of Brownsville, Texas, to undergo a medical procedure. Video showed a gunman dragging people into a white van, a police source with knowledge of the matter said.

Latavia Washington McGee and Eric Williams.WMBF; Courtesy Michelle Williams

Tamaulipas state governor Américo Villarreal said at a press conference that a woman, identified by her 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 and Brownsville.

McGee’s mother said she had spoken to her daughter, and Williams’ wife said he had been shot in the leg but was back in the US.

Zalandria Brown of Florence, South Carolina, said The Associated Press that his younger brother, Zindell, was among the four victims. Aliyah McCleod, McGee’s cousin, identified another member of the group as Shaeed Woodard.

More coverage of the deadly kidnapping in Mexico

McCleod said the group is from South Carolina and was traveling in a rental vehicle with North Carolina plates when it entered Matamoros. Williams’ wife described the group as childhood friends.

A fifth person was on the trip from South Carolina. The traveler left just before the group reached the border because the person did not have the necessary documentation to cross into Mexico, a police official said Wednesday.

That fifth person could be seen riding with the group in video taken by Williams.

A law enforcement official with knowledge of the matter said a woman in the group had been seeking a cosmetic medical procedure. The official said the cartel gunmen targeted the group in a case of mistaken identity.

In its letter, the Gulf cartel said it respects the «life, tranquility and integrity of the innocent» and apologized to «Matamoros society… the American individuals and families affected.»

“In addition, the CDG asks society to remain calm because we are committed to ensuring that these errors caused by indiscipline are not repeated. The culprits will pay, whoever they are,» the letter says.

A 24-year-old man from the state of Tamaulipas, identified only as José N, “was found guarding the victims” and was detained, Villarreal said. It is not clear what charges he faces.

The State Department issued a «not travelAlert for Tamaulipas, noting that organized crimes, including shootings, murders and kidnappings, are common along the border and in Ciudad Victoria.