CNN — The four Americans who authorities say were kidnapped in Mexico on Friday were a tight-knit group of friends traveling from South Carolina so one of them – a mother of six – could undergo a medical procedure across the border, two family members told CNN.

Latavia “Tay” Washington McGee, 33, drove to Mexico with Shaeed Woodard, Zindell Brown and their friend Eric for the procedure, but she never made it to her doctor’s appointment on Friday, her mother Barbara Burgess told CNN.

On Sunday, Burgess said she was informed by the FBI that her daughter had been kidnapped and was in danger. “They said, if she calls me, to call them,” she said.

Mexican authorities are still searching for the missing Americans, who drove into the border city of Matamoros on Friday, where they were fired upon by unidentified gunman and “placed in a vehicle and taken from the scene by armed men,” according to the FBI.

An innocent Mexican bystander was also killed in the encounter, US Ambassador to Mexico Ken Salazar said.

Investigators believe the Americans were targeted by a Mexican cartel that likely mistook them for Haitian drug smugglers, a US official familiar with the ongoing investigation tells CNN.

The US citizens have no concerning criminal history that has been identified by investigators, the official said.

Their abduction highlights the ongoing violence that has plagued some Mexican cities during the long-running Mexican drug war as well as the growing business of “medical tourism.”

Matamoros, a city in the state of Tamaulipas, has a population of more than 500,000 people and is located just across the Rio Grande from Brownsville, Texas. The US State Department has issued a “Level 4: Do Not Travel” advisory for US citizens thinking of going to Tamaulipas, citing crime and kidnapping.

Federal and local Mexican authorities are participating in the effort to locate the missing Americans, Tamaulipas Attorney General Irving Barrios Mojica said Monday.

Investigators have been working to gather surveillance footage, collect ballistics and fingerprint evidence, take biological samples for genetic profiles and process the vehicles involved, Tamaulipas officials said. A joint task force of federal and state agencies also has been created to process information and communicate with US officials, Barrios Mojica said.

The FBI is also requesting the public’s help in finding the Americans and identifying anyone involved in the incident. The agency announced a $50,000 reward for the return of the victims and the arrest of those responsible.

The White House and US State Department are “closely following” the case, spokespeople said in briefings Monday.

Mother was traveling for medical procedure

This trip was the second time Washington McGee, a mother of six children, had gone to Mexico for a medical procedure, her mother said. About two to three years ago, Burgess said, her daughter traveled to the country for a surgery.

Receipts found in the group’s vehicle indicated the Americans were in Mexico for medical procedures, a US official with knowledge of the investigation tells CNN.

Mexican President Andrés Manuel López Obrador said Monday that the group had crossed the border to “buy medicines” and assured the “whole government” is working to resolve the case.

Mexico has become a particularly popular destination for “medical tourism,” attracting travelers who may be seeking cheaper alternatives or medical treatments that are unapproved or unavailable in the US. But the CDC warns the growing trend can carry dangerous risks depending on the destination and facility, including infection and possible post-procedure complications.

The group of missing Americans grew up together in South Carolina and were bonded “like glue,” Brown’s sister Zalandria Brown told CNN. She added that she and her brother are also close.

“Zindell is like my shadow, he’s like my son, he’s like my hip bone. We’re just tight like that,” she said.

CNN has reached out to the FBI, the Tamaulipas Secretary of Public Security’s office and the Mexican Attorney General’s Office for more information.

‘We don’t know if she is dead or alive’

Washington McGee’s aunt, Mary McFadden, told CNN that when the family hadn’t heard from the group of friends by Sunday, they began searching online for any news related to their travel destination. Then, the family saw a video McFadden described as showing her niece being kidnapped.

“We recognized her and her blonde hair,” McFadden said. She said she also recognized her niece’s clothing from a live video Washington McGee had posted to Facebook earlier Friday.

“This happened in plain daylight. We don’t know if she is dead or alive. The last picture we saw, she was walking alive,” McFadden said.

“She is a mother and we need her to come back here for her kids,” she said, adding that Washington McGee’s children range in age from 6 to 18 years old.

A video obtained by CNN shows a woman and other unidentified people being roughly loaded into a white pickup truck. CNN has confirmed the video matches the incident but has not independently confirmed it is the four Americans shown in the video.

The video shows the woman being pulled or pushed onto the bed of the truck by two unidentified people as a third visibly armed man watches. The three men then appear to drag at least two limp people onto the truck bed, the video shows.

Additionally, photos obtained by CNN appear to show fragments of the scene where the situation occurred, including the car believed to have been driven by the Americans crashed with another vehicle before they were taken at gunpoint from the scene.

The US citizens were driving a white minivan with North Carolina plates, according to the FBI in San Antonio.

The FBI would not confirm the authenticity of the photos, but CNN has geolocated the images and confirmed their authenticity with a US official with knowledge of the investigation.

The photos also show a woman looking at and then sitting next to three people lying on the ground outside a white minivan. All the doors of the van are open. It is unclear whether the four people in the photos are the US citizens.

The woman then appears to have been loaded onto the bed of a white pickup truck, beside which several people can be seen lying on the street, the photos show.

One photo shows that an ambulance arrived, but it’s unclear if medical attention was being provided.

CNN’s Josh Campbell, Paul Murphy, Rosa Flores, Sam Fossum, Andi Babineau, Polo Sandoval, Rebekah Reiss and Jorge Engels contributed to this report.