Four Americans kidnapped by heavily armed men in Matamoros, Mexico, had travelled there for cosmetic surgery, relatives told US media.
One of them was to have a tummy tuck - removing abdominal fat - in a border town, said the sister of one of them.
"To see a member of your family thrown in the back of a truck and dragged, it is just unbelievable," Zalandria Brown, told AP about her brother Zindell.
A Mexican woman was killed in the incident.
The FBI has offered a $50,000 (£42,000) reward for the return of the Americans.
The other people in the group have been identified in US media as Latavia McGee, Shaeed Woodard and Eric James Williams.
They were driving through Matamoros - which is directly across the border from the Texas town of Brownsville - in a white minivan with North Carolina licence plates when unidentified gunmen opened fire, the FBI said.
Video shows them being put in a pickup truck by heavily armed men. One is manhandled on to the vehicle while others appear to be unconscious and are dragged to the truck.
Mexican President Andrés Manuel López Obrador described the incident as a "confrontation between armed groups". He said his "entire government" was working to secure the Americans' release.
According to an unnamed US official quoted by CNN, investigators believe a Mexican cartel probably mistook the Americans for Haitian drug smugglers.
Barbara Burgess, Ms McGee's mother, told ABC news that she had warned her daughter not to go but her daughter told her: "Ma, I'll be ok."
On Friday her daughter called her to say she was about to go to the appointment for the tummy tuck operation, also known as an abdominoplasty, which removes abdominal fat.
When Barbara Burgess called her back later that day, the phone went to voicemail.
Ms Brown said her brother, who lives in Myrtle Beach, South Carolina, had been aware that parts of Mexico could be dangerous.
"Zindell kept saying, 'We shouldn't go down'," she told AP, adding that it was "like a bad dream you wish you could wake up from".
Matamoros is in Tamaulipas state, one of six Mexican states that the US state department advises travellers not to visit because of "crime and kidnapping".
Drug cartels control much of the territory and often hold more power than local law enforcement.
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