CNN  — 

Mexican authorities arrested five more people Friday in connection to the kidnappings of four Americans in Matamoros, Mexico, as the bodies of the two Americans killed were returned to US diplomats and questions continued to swirl around last week’s violent abduction.

Six people have been arrested in total, including one on Tuesday, Tamaulipas Attorney General Irving Barrios Mojica said Friday.

“The Tamaulipas Attorney General’s Office (#FGJT) performed an arrest warrant against 5 people linked to the events of March 3 in Matamoros, for the crimes of aggravated kidnapping and intentional simple homicide. One more person who was arrested in recent days, was linked to the process,” Barrios Mojica tweeted.

The case remained “very confusing” to investigators, who were still obtaining information on last week’s kidnapping and considering all angles, a Tamaulipas Prosecutor’s Office official familiar with the investigation told CNN before news broke of the arrests.

Earlier, a cartel apologized for carrying out what one victim’s father has called “a senseless crime” that also left one Mexican woman dead.

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.

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 who has been detained was undertaking “surveillance functions of the victims,” Tamaulipas Gov. Américo Villarreal said Tuesday, identifying the individual as 24-year-old Jose “N.”

(From left to right) LaTavia Washington McGee and Eric Williams survived the kidnapping, while Shaeed Woodard and Zindell Brown were killed.

The Tamaulipas attorney general’s office identified the person arrested Tuesday as Jose Guadalupe “G.” A judge ordered him to be temporarily detained for five months for the investigation to be carried out, the attorney general’s office said. Officials would not confirm whether the man has any affiliations with criminal organizations.

Meanwhile, the bodies of two Americans killed – Shaeed Woodard and Zindell Brown – were delivered to US diplomatic authorities Thursday after undergoing forensic examination, Barrios Mojica said in a tweet.

“I’ve tried to make sense out of it and