A powerful Mexican cartel has allegedly taken responsibility for the kidnapping of four U.S. citizens, and killing of two of them, in the Mexican border city of Matamoros.

Five men were found tied up near a pickup truck on Thursday morning. ABC News has geolocated that location as the same area where the U.S. citizens were kidnapped on Friday.

A handwritten note was found placed on the windshield of the truck, whose author or authors say they belong to the Gulf Cartel, the dominant organized crime group in this part of Mexico.

The note said some Gulf Cartel members were responsible for the kidnapping and killings and apologized for their actions.

"We have decided to deliver those involved and directly responsible," the note said, presumably referring to the five men found tied up at the scene.

Multiple different law enforcement agencies along with members of the Mexican army responded to the scene.

Three sources confirm to ABC News that the five men were taken into custody by Tamaulipas State Police, where they remain.

A source close to the investigation said they believe the note left on the windshield to be legitimate.

A source close to the investigation believes that state police and other Mexican law enforcement want to verify the identities of those five men arrested and be sure they were in fact connected to the kidnappings before commenting publicly.

ABC News has reached out to the FBI for comment.