A note from the assassins' boss was left with the five men, apologizing for the killings and suggesting that they were happy to hand over those responsibe for the attack, according to the Daily Mail.
The five suspects could be seen smirking when they were apprehended in Matamoros on Wednesday night.
The narco bosses claimed that the assassins, who were apparently members of the Scorpions splinter group, did not respect "cartel rules" and condemned the attacks, adding that "the CDG [Cartel del Golfo] has always respected the life and integrity of the innocent."
Questions have been raised about the precise nature of why the Americans were in the cartel-ridden city in the first place. Though the survivors' families have claimed that they were there for one of them to have a tummy tuck surgery, the Daily Mail has recently reported that, due to previous criminal records involving drugs, narcotics may have had a role to play.
The four Americans - Latavia McGee, Eric Williams, Shaeed Woodard, and Zindell Brown - all have an extensive history with drugs, according to the report.
McGee was charged with "unlawful conduct towards a child" after her daughter tested positive for amphetamines; Williams had been previously caught "distributing crack near a school"; Woodard was charged with the "manufacturing and possession" of illicit drugs and ultimately pleaded guilty; and Brown was charged for "possession of marijuana or hashish."
The Mexican government is also investigating whether the kidnapping could have anything to do with direct links "to drug trafficking."
The Spanish letter left with the alleged assassins reads, in translation: "The Gulf Cartel's Scorpions Group strongly condemns the events of last Friday, March 3 where unfortunately an 'innocent' working mother died and 4 American citizens were 'kidnapped' of which 2 died."
The upper brass of the Gulf Cartel said that the suspects had acted on their own.
"And for this reason, we have decided to hand over those involved and directly responsible for the facts who at all times acted under their own determination and indiscipline and against the rules that 'The CDG' has always operated, respecting the life and integrity of the innocent."
"The CDG apologizes to Matamoran society, the relatives of Mrs. Arely, the American individuals and families affected."
"The CDG asks society to be calm because we are committed to not repeating those errors caused by indiscipline and whoever is responsible will pay!!"
It was reported that the five suspects have yet to be brought up on charges, according to a spokesman for the Tamaulipas State Attorney General's Office.