Four U.S. citizens have reportedly been kidnapped and two of them killed by a powerful Mexican cartel in the border city of Matamoros.

On Thursday, five men were discovered tied up near a pickup truck in the same area where the U.S. citizens were taken.

A handwritten note was left on the truck’s windshield, claiming responsibility for the crime and stating that the Gulf Cartel, the predominant criminal organization in the region, was behind it. ABC News has geolocated the location as the same area where the kidnapping occurred on Friday.

According to a note found at the scene, the Gulf Cartel claimed responsibility for the kidnapping and killings and offered an apology. The note stated that they had identified those involved and responsible for the incident, likely referring to the five individuals who were found bound at the location.

Several law enforcement agencies and soldiers from the Mexican army arrived at the location to respond to the situation.

According to three sources, the Tamaulipas State Police detained the five men who were allegedly involved in the incident, and they are currently in custody. Several individuals familiar with the case believe that the note left on the windshield is authentic. However, neither ABC News nor U.S. officials have been able to independently confirm the note’s authenticity.

According to a source familiar with the investigation, Mexican authorities reportedly discovered two firearms, an AK-47 and an AR-15, as well as four AK-47 magazines, three AR-15 magazines, and various cartridges of different calibers, inside the truck when they apprehended the five men. The information was obtained from a copy of the police report obtained by ABC News.

Police found the men inside the truck with their hands tied with belts, along with the note, according to the police report.

An individual with knowledge of the investigation suggests that Mexican law enforcement, including state police, are working to confirm the identities of the five detained men to ensure that they were involved in the kidnappings before making any public statements. ABC News has reached out to the FBI for a statement.

The four abducted Americans, Eric James Williams, Zindell Brown, and cousins Latavia “Tay” McGee and Shaeed Woodard, drove into Matamoros, located in Tamaulipas, northeast Mexico, just south of Brownsville, Texas. McGee traveled from South Carolina to Mexico for a cosmetic medical procedure.

Soon after crossing the border, “unidentified gunmen fired upon the passengers in the vehicle,” and then put the four Americans in another car and fled, the U.S. Embassy in Mexico City said.

According to a source familiar with the investigation, Mexican investigators think that the kidnappers may have mistakenly assumed that the Americans were rival human traffickers.

Mexican officials reported on Tuesday that the two surviving Americans, McGee and Williams, were discovered in a wooden house in the Lagunona region outside of Matamoros. Williams had gunshot wounds in both legs, while McGee was mostly unscathed, according to family members.

According to a source close to the investigation, one of the deceased was discovered inside the house while the other was found outside. The Americans were taken to different locations, including a medical clinic, over the course of three days to prevent any rescue attempts and create confusion, said Américo Villarreal, the governor of Tamaulipas.

Mexican officials stated that they have arrested a 24-year-old suspect who was responsible for taking care of the victims inside the house where they were discovered. The two survivors were rescued and taken to a hospital in Brownsville for treatment. Meanwhile, the bodies of the two Americans who were killed were repatriated to the U.S. on Thursday. Additionally, the Attorney General’s Office of Tamaulipas confirmed that a Mexican woman had also died during the incident.