The cartel believed responsible for the armed kidnapping last week that killed two American tourists and a Mexican woman in Matamoros, Mexico, issued an apology letter and handed over five of their members to local authorities, according to images circulating online and a version of the letter obtained by CNN from an official familiar with the ongoing investigation.

The source told CNN that investigators believe the letter to be authentic.

“The [Gulf Cartel] apologizes to the society of Matamoros, the relatives of Ms. Areli, and the affected American people and families,” reads the handwritten letter, referring to a Mexican woman who was also killed in the shootout.

The bodies of the two Americans killed in the kidnapping were delivered Thursday to US diplomatic authorities, according to a Mexican official. Two other American tourists survived the kidnapping.

In one of the photos posted online, five men can be seen lying face down and restrained on the pavement with their shirts pulled over their faces. Uniformed authorities are seen standing over them.

“The Gulf Cartel, Scorpion Group, strongly condemns the events of last Friday,” the letter continues, referring to a division of the cartel. “For this reason, we decided to hand over those directly involved and responsible for the acts, who at all times acted under their own determination and indiscipline and against the rules in which the [Gulf Cartel] always operates.”

CNN cannot confirm the authenticity of the photos and has asked Mexican and US authorities for comment.

It is common for Mexican cartels, especially in the northeast of the country, to release messages to the authorities or rival groups in the aftermath of high-profile incidents, according to Guadalupe Correa-Cabrera, a professor at George Mason University who studies the cartels.

The official who confirmed the apparent legitimacy of the letter said Mexican and US law enforcement officials investigating the kidnapping strongly doubt the sincerity of the group’s apology. But they suspect the apology was issued after the attack brought considerable public attention and scrutiny onto the actions of the cartel.

The development comes after a local leader of the Gulf Cartel, wanted for past kidnappings, was arrested by Mexican authorities in Reynosa, according to a US official briefed on the apprehension.

Any connection to last week’s armed kidnapping of four Americans was unclear. But, as CNN has reported, the official believes members of the Gulf Cartel attacked the Americans in Matamoros in a case of mistaken identity.

The local cartel leader, Ernesto Sanchez-Rivera, is also known to have ties to the Jalisco New Generation Cartel, the source added.

CNN has reached out to the local prosecutor for more information on the apprehension but has not yet received a response.

The bodies of Shaeed Woodard and Zindell Brown, the two Americans killed in the kidnapping, have been delivered to US diplomatic authorities after two survivors of the attack returned to the US for treatment at a hospital, Tamaulipas Attorney General Irving Barrios Mojica said in a tweet Thursday

Mexico’s forensic studies on the two American victims have also concluded, Barrios added.

The remains of Woodard and Brown likely will be transported to a funeral home in Brownsville, Texas, a US official familiar with the investigation said. The repatriation would come two days after the bodies were discovered alongside their two surviving friends in a house around the Mexican city of Matamoros.

Autopsies were completed Wednesday morning in Mexico, an official from the Tamaulipas Prosecutor’s Office told CNN, though Mexican authorities have not released causes of death. Second autopsies will be performed in the US, the US official said.

CNN has reached out to the US State Department about the repatriation of remains.

The deceased were part of a group of four friends from South Carolina who had driven Friday into Matamoros so one of them, Latavia Washington McGee, could undergo a medical procedure, two family members told CNN. But their trip was violently interrupted when unidentified gunmen fired on their van, then loaded the Americans into a vehicle and drove them away, the FBI said.

An innocent Mexican bystander was also killed by a stray bullet almost a block and a half from where the Americans were kidnapped, according to Tamaulipas Gov. Américo Villarreal.

James Woodard, Shaeed Woodard’s father, said it was difficult to see the video of his son’s abduction on television.

“That was so hard for me to see those videos and see him dragged and thrown on the back of the vehicle,” the elder Woodard said on Thursday, which would have been his son’s 34th birthday. “He was a baby and for him to be taken from me like that was very hurtful. My family is hurt real bad because he was so lovable. He had the biggest heart.”

Survivor Eric Williams was shot three times in the legs, his wife Michele Williams told CNN. When he and McGee were discovered alive Tuesday, Williams was taken to a hospital in Texas for surgery, she said.

Washington McGee was also taken to the hospital, her mother, Barbara Burgess, told CNN, though Mexican authorities said she was uninjured.

“She watched them die,” Burgess said, recounting what Washington McGee told her about the kidnapping. “They were driving through and a van came up and hit them, and that’s when they started shooting at the car, shooting inside the van. … She said the others tried to run and they got shot at the same time.”

Washington McGee and Brown are cousins who were raised together as closely as siblings, Burgess said.

“He was a good person, and I miss him,” Burgess said of Brown. “I loved him. (There’s) nothing I wouldn’t do for him.”

How Mexico has responded

Investigators believe the group was targeted by a Mexican cartel who mistook them for Haitian drug smugglers, a US official familiar with the investigation told CNN on Monday, and the kidnapping has renewed attention to efforts by US and Mexican officials to combat organized crime in Mexico.

However, during a Wednesday news briefing held by Mexican President Andrés Manuel López Obrador, a government-sponsored fact-checking agency claimed reports of the Americans being mistaken for Haitian drug traffickers are false. The president said “adversaries” in Mexico and the US are attempting to make a “scandal” of the case.

CNN has reached out to investigators in the US and Mexico, as well as the fact-checking agency.

The Tamaulipas prosecutor’s office, meanwhile, has located an ambulance and a clinic in Matamoros connected to the kidnapping.

The ambulance was used to transport the kidnap victims to a clinic, where they were treated, before officials later discovered them in a “wooden house.”

In a statement on Thursday, the prosecutor’s office said an investigation revealed that the clinic was used to provide first aid care to the US citizens: “For this purpose, they were taken aboard the ambulance, which was seized as evidence and to continue the investigation.

“The ambulance was located, which, according to the testimony of the victims, was used to take them to the clinic,” the statement said. No one was detained after officials located the clinic and ambulance, the statement said.

The fatal kidnapping – and the possibility it was carried out by a cartel – has brought increased attention to ongoing efforts by US and Mexican officials to curb cartel activity that is a primary driver of the fentanyl trade between the countries. Mexico is the “dominant source” of fentanyl in the US, according to a government report released last year.

A US delegation traveled to Mexico this week to “discuss our governments’ ongoing cooperation in combating illicit fentanyl,” a national security council spokesman told CNN on Wednesday.

The delegation plans to address the kidnapping and discuss a “fundamental strategy to attack the cartels,” Department of Homeland Security Secretary Alejandro Mayorkas said Wednesday.

Timeline of the kidnapping

The kidnapping of the four friends on Friday spurred a days-long investigation by local and federal Mexican officials, who say they were in almost-constant contact with US authorities until the two survivors and the victims’ bodies were finally discovered.

The four friends had booked a hotel in Brownsville, Texas, and were planning to drive to a doctor’s office in Matamoros on Friday for Washington McGee to undergo a medical procedure, a close friend who did not want to be identified told CNN.

At about 9:18 a.m. Friday, the group crossed into Matamoros, Tamaulipas Governor Américo Villarreal said. But on their way to the clinic, the group became lost and were struggling to contact the doctor’s office for directions due to a poor phone signal, the close friend said.

Suddenly, another vehicle collided into the group’s van and gunmen began shooting at the group, sending some of the friends running, according to Burgess, who recounted her daughter’s experience. “They all got shot at the same time,” she said.

A video obtained by CNN shows Washington McGee being shoved onto the bed of a white pickup truck by a group of armed men, who then begin dragging at least two other limp bodies into the truck. Burgess, when asked about the video, said her daughter was treated “like trash.”

The Americans were then taken from the scene in the vehicle, according to an FBI account of the kidnapping.

Over the next few days, the groups was moved to several different locations to “create confusion and avoid rescue efforts,” Villarreal said.

Mexican investigators, meanwhile, were searching for the missing group, sifting through surveillance footage and processing the vehicles and ballistics found at the scene, officials said.

After noticing the Americans’ van had North Carolina license plates, Mexican authorities reached out to US officials, who were able to run the plates, according to Barrios Mojica, the Tamaulipas attorney general. They were also able to identify the gunmen’s truck, he said.

“Several searches” were then initiated across multiple agencies, and the group was ultimately found in the wooden house in or near Matamoros on Tuesday morning, Villarreal said.

Though US law enforcement were not involved in the search on the ground, federal and local agencies in Mexico were cooperating in the effort and a joint task force was created to communicate with US officials, Barrios Mojica said. (CNN)

•PHOTO: Cheryl Orange traveled with her friends from South Carolina to Texas, but didn't cross the border into Mexico with them because she didn't have her ID. She recounts the moments that lead to their disappearance with CNN's Anderson Cooper.

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