A week after the violent kidnappings of four Americans in Mexico, investigators are still working to piece together how and why the abduction unfolded, even after a cartel apologized for carrying out what one victim’s father has called “a senseless crime” that left two Americans and one Mexican woman dead.

The case remains “very confusing” to investigators, who are still obtaining information on the kidnapping last Friday and considering all angles, a Tamaulipas Prosecutor’s Office official familiar with the investigation told CNN.

Though authorities have not publicly named any suspects, an apology letter was issued Thursday by the Gulf Cartel, which is believed to be responsible for the kidnappings, and the group handed over five of its 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.

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

“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 by a stray bullet in the shootout.

Though investigators believe the letter to be authentic, Mexican and US law enforcement officials participating in the investigation strongly doubt the sincerity of the group’s apology, the official who shared the letter with CNN said.

One person has been detained in connection with the Americans’ deaths who was undertaking “surveillance functions of the victims,” Tamaulipas Gov. Américo Villarreal said Tuesday, identifying the individual as 24-year-old Jose “N.”

Officials would not confirm whether the man has any affiliations with criminal organizations.

The bodies of two Americans killed in the kidnapping – Shaeed Woodard and Zindell Brown – were delivered to US diplomatic authorities Thursday after undergoing forensic examination, Tamaulipas Attorney General Irving Barrios said in a tweet.

“I’ve tried to make sense out of it and tried to be strong about it,” Woodard’s father, James Woodard, told reporters on Thursday, which would have been his son’s 34th birthday.

“It just was a senseless crime.”

The two survivors – Latavia Washington McGee and Eric Williams – returned to the US on Tuesday to be treated in a hospital. Williams, who had been shot three times in his legs, has since undergone two surgeries and had rods placed in his legs, his wife said on a GoFundMe page to raise money for Williams’s medical and living expenses.

A fifth American group member, Cheryl Orange, planned to travel with the group on the day of the kidnapping but had to stay behind because she did not have proper identification to cross the border. She told CNN that she has battled the guilt of narrowly missing the attack.

“I beat myself up in the beginning about that and I have everybody telling me that I need to be grateful. I really wish I was by Tay’s side,” Orange said, referring to her “best friend,” Washington McGee by her nickname “Tay.”

The tight-knit group had traveled from South Carolina to Matamoros so that Washington McGee could undergo a medical procedure.

But the friends were violently intercepted by gunmen who fired into the Americans’ van, roughly loaded them into the back of a truck and took them away, according to Washington McGee’s mother and a video of the encounter.

The victims were shuttled to multiple locations before they were found in a house around Matamoros on Tuesday, Villarreal said. Tamaulipas prosecutors have since found an ambulance that was used to transport the victims to first aid treatment at a clinic, which authorities have also located, the prosecutor’s office said in a statement.

‘It was music to my ears to hear her voice’

As the group of friends crossed into Matamoros last Friday, Orange stayed behind at their hotel in Brownsville, Texas, becoming increasingly concerned as evening came and the friends hadn’t returned, she told CNN’s Anderson Cooper Thursday.

“I said something’s not right,” Orange said. She reached out to her boyfriend and Washington McGee’s brother to say she was getting worried.

When it came time for Orange to check out of their hotel the next morning, there was still no sign of Washington McGee and the others, Orange said.

At that point, she became so concerned that she decided to call the police.

Orange reported the group missing on Saturday to Brownsville Police, according to a police report. The report states that police checked a local jail to make sure that no one in the party had been taken into custody, but no other action was taken.

Eventually, Orange saw the video of the kidnapping that was circulating online, showing Washington McGee being shoved into the back of a truck by armed gunmen and the other victims’ bodies being dragged in beside her.

“My body clenched up. I dropped the phone. My stomach was in knots and I just began praying for the return of them,” she said of seeing the video.

Upon finally hearing Washington McGee’s voice after she was discovered alive, Orange was able to feel some relief.

“It put me at ease a little bit. It was music to my ears to hear her voice,” she said.

Meanwhile, the families of Woodard and Brown are left to grapple with the loss of their loved ones.

“That was hard for me to see those videos and see him dragged and thrown on the back of a vehicle. It’s like god was preparing me already to know that it was probably the worst,” Woodard’s father said of watching the video of the kidnapping.

Woodard had accompanied his cousin, Washington McGee to Mexico for her procedure, but also to celebrate his upcoming 34th birthday, his father said.

He described his son as a “sweetheart” and a “loving person.”

“If you told me this day was coming I would have never believed it,” James Woodard said. He later added, “A parent never expects to lose a child.”

Apology letter follows arrest of cartel leader

US and Mexican law enforcement officials suspect the Gulf Cartel’s apology letter was issued after the kidnapping exposed the cartel to considerable public attention and scrutiny of its actions, according to the US official who confirmed the letter’s authenticity.

In its letter, the cartel apologized to “the society of Matamoros, the relatives of Ms. Areli, and the affected American people and families,” referring to the Mexican woman who was killed by a stray bullet.

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 apology came after the arrest of a local Gulf Cartel leader, who was wanted for past kidnappings, in the city of Reynosa, about 55 miles west of Matamoros, according to a US official briefed on the apprehension.

Any connection to last week’s kidnapping of the Americans is unclear. But, as CNN has reported, the official believes members of the Gulf Cartel attacked the Americans in Matamoros, after mistaking them for Haitian drug smugglers.

The local cartel leader, Ernesto Sanchez-Rivera, is also known as “Metro 22” and “La Mierda” and is known to also 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 kidnapping of Americans has brought increased scrutiny to efforts to reign in cartel violence in Mexico, including from Republican lawmakers in the US who have called for designating cartels as terrorist organizations and signaled their plans to file legislation allowing the US military to operate in Mexico.

The pressure from Republicans has been met with a swift rebuke from Mexican President Andrés Manuel López Obrador, who says the actions would infringe on Mexican sovereignty.