Mexican drug cartel apologises for kidnapping and killing Americans and hands over gang members responsible for 'lack of discipline'

A Mexican drug cartel blamed for abducting four Americans and killing two of them has apparently condemned the violence and said it has turned over the members responsible to the authorities.

The Scorpions faction of the Gulf cartel apologised to the four Americans and their families, the residents of Matamoros where they were kidnapped and a Mexican woman who died in a shooting at the time they were abducted, according to a letter obtained by the Associated Press.

"We have decided to turn over those who were directly involved and responsible in the events, who at all times acted under their own decision-making and lack of discipline," reads the letter, which was handed to the agency by an unnamed police source.

It said those individuals had gone against the cartel's rules, which include "respecting the life and well-being of the innocent".

A photo of five men bound and face down on the pavement accompanied the letter.

Another unnamed state security official told the agency five men had been found tied up inside one of the vehicles authorities had been searching for, along with the letter.

State officials did not immediately publicly announce they had new suspects in custody.

Last Friday, the Americans travelled from South Carolina so one of them could get a tummy tuck from a doctor in Matamoros.

Shortly after reaching Mexico they were fired upon before being loaded into the back of a pickup truck by gunmen.

A Mexican woman was killed in the shooting.

It is unclear if the Americans were ambushed or caught in the crossfire between two groups of rival drug cartel factions.

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Two kidnapped Americans found dead

Authorities located them on the outskirts of the city on Tuesday morning, guarded by a man who was arrested.

Two of the Americans were dead, while another was wounded and the other was unharmed.

The Tamaulipas state prosecutor's office said it had seized an ambulance and was examining a medical clinic in Matamoros that was allegedly used to treat the Americans after the shooting. No arrests were made.

The Americans told investigators they had been taken to the clinic in an ambulance to provide first aid.

Matamoros is dominated by factions of the powerful Gulf drug cartel, who often fight among themselves.

Thousands of Mexicans have gone missing in Tamaulipas state alone due to the violence.